diff --git a/src/cooler/cli/cload.py b/src/cooler/cli/cload.py
index b2fa749..ded4b19 100644
--- a/src/cooler/cli/cload.py
+++ b/src/cooler/cli/cload.py
@@ -372,12 +372,6 @@ def pairix(
 @click.option(
     "--pos2", "-p2", help="pos2 field number (one-based)", type=int, required=True
 )
-@click.option(
-    "--chunksize",
-    help="Number of input lines to load at a time",
-    type=int,
-    default=int(15e6),
-)
 @click.option(
     "--zero-based",
     "-0",
@@ -433,6 +427,28 @@ def pairix(
 #          "specify at least one input field for aggregation as an alternative.",
 #     is_flag=True,
 #     default=False)
+@click.option(
+    "--chunksize",
+    "-c",
+    help="Size in number of lines/records of data chunks to read and process "
+    "from the input stream at a time. These chunks will be saved as temporary "
+    "partial coolers and then merged.",
+    type=int,
+    default=15_000_000,
+)
+@click.option(
+    "--mergebuf",
+    help="Total number of pixel records to buffer per epoch of merging data. "
+    "Defaults to the same value as `chunksize`.",
+    type=int,
+)
+@click.option(
+    "--max-merge",
+    help="Maximum number of chunks to merge in a single pass.",
+    type=int,
+    default=200,
+    show_default=True,
+)
 @click.option(
     "--temp-dir",
     help="Create temporary files in a specified directory. Pass ``-`` to use "
@@ -445,13 +461,6 @@ def pairix(
     is_flag=True,
     default=False,
 )
-@click.option(
-    "--max-merge",
-    help="Maximum number of chunks to merge before invoking recursive merging",
-    type=int,
-    default=200,
-    show_default=True,
-)
 @click.option(
     "--storage-options",
     help="Options to modify the data filter pipeline. Provide as a "
@@ -478,12 +487,13 @@ def pairs(
     cool_path,
     metadata,
     assembly,
-    chunksize,
     zero_based,
     comment_char,
     input_copy_status,
     no_symmetric_upper,
     field,
+    chunksize,
+    mergebuf,
     temp_dir,
     no_delete_temp,
     max_merge,
@@ -501,6 +511,8 @@ def pairs(
 
     """
     chromsizes, bins = parse_bins(bins)
+    if mergebuf is None:
+        mergebuf = chunksize
 
     symmetric_upper = not no_symmetric_upper
     tril_action = None
@@ -639,7 +651,7 @@ def pairs(
         dtypes=output_field_dtypes,
         metadata=metadata,
         assembly=assembly,
-        mergebuf=chunksize,
+        mergebuf=mergebuf,
         max_merge=max_merge,
         temp_dir=temp_dir,
         delete_temp=not no_delete_temp,
diff --git a/src/cooler/cli/load.py b/src/cooler/cli/load.py
index 837695c..6717ac4 100644
--- a/src/cooler/cli/load.py
+++ b/src/cooler/cli/load.py
@@ -46,16 +46,6 @@
     type=str,
     multiple=True,
 )
-@click.option(
-    "--chunksize",
-    "-c",
-    help="Size (in number of lines/records) of data chunks to read and process "
-    "from the input file at a time. These chunks will be saved as "
-    "temporary partial Coolers and merged at the end. Also specifies the "
-    "size of the buffer during the merge step.",
-    type=int,
-    default=int(20e6),
-)
 @click.option(
     "--count-as-float",
     is_flag=True,
@@ -99,12 +89,34 @@
     "distinct, use the ``--no-symmetric-upper`` option instead. ",
     show_default=True,
 )
+@click.option(
+    "--chunksize",
+    "-c",
+    help="Size in number of lines/records of data chunks to read and process "
+    "from the input stream at a time. These chunks will be saved as temporary "
+    "partial coolers and then merged.",
+    type=int,
+    default=20_000_000,
+)
+@click.option(
+    "--mergebuf",
+    help="Total number of records to buffer per epoch of merging data. Defaults "
+    "to the same value as `chunksize`.",
+    type=int,
+)
 @click.option(
     "--temp-dir",
     help="Create temporary files in a specified directory. Pass ``-`` to use "
     "the platform default temp dir.",
     type=click.Path(exists=True, file_okay=False, dir_okay=True, allow_dash=True),
 )
+@click.option(
+    "--max-merge",
+    help="Maximum number of chunks to merge in a single pass.",
+    type=int,
+    default=200,
+    show_default=True,
+)
 @click.option(
     "--no-delete-temp",
     help="Do not delete temporary files when finished.",
@@ -133,13 +145,15 @@ def load(
     format,
     metadata,
     assembly,
-    chunksize,
     field,
     count_as_float,
     one_based,
     comment_char,
     input_copy_status,
     no_symmetric_upper,
+    chunksize,
+    mergebuf,
+    max_merge,
     temp_dir,
     no_delete_temp,
     storage_options,
@@ -178,6 +192,8 @@ def load(
     """
     logger = get_logger(__name__)
     chromsizes, bins = parse_bins(bins_path)
+    if mergebuf is None:
+        mergebuf = chunksize
 
     symmetric_upper = not no_symmetric_upper
     tril_action = None
@@ -334,10 +350,11 @@ def load(
         dtypes=output_field_dtypes,
         metadata=metadata,
         assembly=assembly,
-        mergebuf=chunksize,
-        ensure_sorted=False,
+        mergebuf=mergebuf,
+        max_merge=max_merge,
         temp_dir=temp_dir,
         delete_temp=not no_delete_temp,
+        ensure_sorted=False,
         # boundscheck=True,
         # dupcheck=True,
         triucheck=True if symmetric_upper else False,
diff --git a/src/cooler/reduce.py b/src/cooler/reduce.py
index 35442ba..77729d4 100644
--- a/src/cooler/reduce.py
+++ b/src/cooler/reduce.py
@@ -44,68 +44,91 @@
 
 
 def merge_breakpoints(
-    indexes: list[np.ndarray],
-    maxbuf: int
+    indexes: list[np.ndarray | h5py.Dataset],
+    bufsize: int
 ) -> tuple[np.ndarray, np.ndarray]:
     """
-    Partition k offset arrays for performing a k-way external merge, such that
-    no single merge pass loads more than ``maxbuf`` records  into memory, with
-    one exception (see Notes).
+    Given ``k`` bin1_offset indexes, determine how to partition the data from
+    ``k`` corresponding pixel tables for a k-way merge.
+
+    The paritition is a subsequence of bin1 IDs, defining the bounds of chunks
+    of data that will be loaded into memory from each table in a single "epoch"
+    of merging data. The bounds are calculated such that no single epoch will
+    load more than ``bufsize`` records into memory.
+
+    However, the ``bufsize`` condition is not guaranteed and a warning will be
+    raised if it cannot be satisfied for one or more epochs (see Notes).
 
     Parameters
     ----------
-    indexes : sequence of 1D arrays of equal length
-        These offset-array indexes map non-negative integers to their offset
-        locations in a corresponding data table
-    maxbuf : int
-        Maximum cumulative number of records loaded into memory for a single
-        merge pass
+    indexes : sequence of 1D array-like of equal length
+        Offset arrays that map bin1 IDs to their offset locations in a
+        corresponding pixel table.
+    bufsize : int
+        Maximum number of pixel records loaded into memory in a single merge
+        epoch.
 
     Returns
     -------
-    breakpoints : 1D array
-        breakpoint locations to segment all the offset arrays
-    cum_offset : 1D array
-        cumulative number of records that will be processed at each breakpoint
+    bin1_partition : 1D array
+        Bin1 IDs defining where to partition all the tables for merging.
+    cum_nrecords : 1D array
+        Cumulative number of records (from all pixel tables combined) that will
+        be processed at each epoch.
 
     Notes
     -----
-    The one exception to the post-condition is if any single increment of the
-    indexes maps to more than ``maxbuf`` records, these will produce
-    oversized chunks.
-
+    The one exception to the post-condition is when a single bin1 increment in
+    a table contains more than ``bufsize`` records.
     """
-    # k = len(indexes)
-
-    # the virtual cumulative index if no pixels were merged
-    cumindex = np.vstack(indexes).sum(axis=0)
-    cum_start = 0
-    cum_nnz = cumindex[-1]
-    # n = len(cumindex)
-
-    breakpoints = [0]
-    cum_offsets = [0]
+    # This is a "virtual" cumulative index if all the tables were concatenated
+    # and sorted but no pixel records were aggregated. It helps us track how
+    # many records would be processed at each merge epoch.
+    # NOTE: We sum these incrementally in case the indexes are lazy to avoid
+    # loading all indexes into memory at once.
+    combined_index = np.zeros(indexes[0].shape)
+    for i in range(len(indexes)):
+        combined_index += indexes[i]
+    combined_start = 0
+    combined_nnz = combined_index[-1]
+
+    bin1_partition = [0]
+    cum_nrecords = [0]
     lo = 0
     while True:
-        # find the next mark
-        hi = bisect_right(cumindex, min(cum_start + maxbuf, cum_nnz), lo=lo) - 1
+        # Find the next bin1 ID from the combined index
+        hi = bisect_right(
+            combined_index,
+            min(combined_start + bufsize, combined_nnz),
+            lo=lo
+        ) - 1
+
         if hi == lo:
-            # number of records to nearest mark exceeds `maxbuf`
-            # check for oversized chunks afterwards
+            # This means number of records to nearest mark exceeds `bufsize`.
+            # Check for oversized chunks afterwards.
             hi += 1
 
-        breakpoints.append(hi)
-        cum_offsets.append(cumindex[hi])
+        bin1_partition.append(hi)
+        cum_nrecords.append(combined_index[hi])
 
-        if cumindex[hi] == cum_nnz:
+        if combined_index[hi] == combined_nnz:
             break
 
         lo = hi
-        cum_start = cumindex[hi]
+        combined_start = combined_index[hi]
 
-    breakpoints = np.array(breakpoints)
-    cum_offsets = np.array(cum_offsets)
-    return breakpoints, cum_offsets
+    bin1_partition = np.array(bin1_partition)
+    cum_nrecords = np.array(cum_nrecords)
+
+    nrecords_per_epoch = np.diff(cum_nrecords)
+    n_over = (nrecords_per_epoch > bufsize).sum()
+    if n_over > 0:
+        warnings.warn(
+            f"{n_over} merge epochs will require buffering more than {bufsize} "
+            f"pixel records, with as many as {nrecords_per_epoch.max():g}."
+        )
+
+    return bin1_partition, cum_nrecords
 
 
 class CoolerMerger(ContactBinner):
@@ -117,12 +140,12 @@ class CoolerMerger(ContactBinner):
     def __init__(
         self,
         coolers: list[Cooler],
-        maxbuf: int,
+        mergebuf: int,
         columns: list[str] | None = None,
         agg: dict[str, Any] | None = None
     ):
         self.coolers = list(coolers)
-        self.maxbuf = maxbuf
+        self.mergebuf = mergebuf
         self.columns = ["count"] if columns is None else columns
         self.agg = {col: "sum" for col in self.columns}
         if agg is not None:
@@ -145,18 +168,23 @@ def __init__(
                     raise ValueError("Coolers must have same bin structure")
 
     def __iter__(self) -> Iterator[dict[str, np.ndarray]]:
-        indexes = [c._load_dset("indexes/bin1_offset") for c in self.coolers]
-        breakpoints, cum_offsets = merge_breakpoints(indexes, self.maxbuf)
-        chunksizes = np.diff(cum_offsets)
-        if chunksizes.max() > self.maxbuf:
-            warnings.warn(f"Some merge passes will use more than {self.maxbuf} pixels")
+        # Load bin1_offset indexes lazily.
+        indexes = [c.open("r")["indexes/bin1_offset"] for c in self.coolers]
+
+        # Calculate the common partition of bin1 offsets that define the epochs
+        # of merging data.
+        bin1_partition, cum_nrecords = merge_breakpoints(indexes, self.mergebuf)
+        nrecords_per_epoch = np.diff(cum_nrecords)
+        logger.info(f"n_merge_epochs: {len(nrecords_per_epoch)}")
+        logger.debug(f"bin1_partition: {bin1_partition}")
+        logger.debug(f"nrecords_per_merge_epoch: {nrecords_per_epoch}")
+
         nnzs = [len(c.pixels()) for c in self.coolers]
         logger.info(f"nnzs: {nnzs}")
 
         starts = [0] * len(self.coolers)
-        for bp in breakpoints[1:]:
-            stops = [index[bp] for index in indexes]
-            logger.info(f"current: {stops}")
+        for bin1_id in bin1_partition[1:]:
+            stops = [index[bin1_id] for index in indexes]
 
             # extract, concat
             combined = pd.concat(
@@ -178,6 +206,7 @@ def __iter__(self) -> Iterator[dict[str, np.ndarray]]:
 
             yield {k: v.values for k, v in df.items()}
 
+            logger.info(f"records consumed: {stops}")
             starts = stops
 
 
@@ -265,7 +294,7 @@ def merge_coolers(
 
     bins = clrs[0].bins()[["chrom", "start", "end"]][:]
     assembly = clrs[0].info.get("genome-assembly", None)
-    iterator = CoolerMerger(clrs, maxbuf=mergebuf, columns=columns, agg=agg)
+    iterator = CoolerMerger(clrs, mergebuf=mergebuf, columns=columns, agg=agg)
 
     create(
         output_uri,
diff --git a/tests/test_create_ingest.py b/tests/test_create_ingest.py
index a0e5752..99a8f9c 100644
--- a/tests/test_create_ingest.py
+++ b/tests/test_create_ingest.py
@@ -301,17 +301,18 @@ def test_cload_pairs(bins_path, pairs_path, ref_path):
     kwargs = dict(
         metadata=None,
         assembly="hg19",
-        chunksize=int(15e6),
         zero_based=False,
         comment_char="#",
         input_copy_status="unique",
         no_symmetric_upper=False,
         field=(),
+        chunksize=int(15e6),
+        mergebuf=int(20e6),
+        max_merge=200,
         temp_dir=None,
         no_delete_temp=False,
         storage_options=None,
         no_count=False,
-        max_merge=200,
         chrom1=2,
         pos1=3,
         chrom2=4,
@@ -342,16 +343,17 @@ def test_cload_field(bins_path, pairs_path):
     kwargs = dict(
         metadata=None,
         assembly="toy",
-        chunksize=10,
         zero_based=False,
         comment_char="#",
         input_copy_status="unique",
         no_symmetric_upper=False,
+        chunksize=10,
+        mergebuf=10,
+        max_merge=200,
         temp_dir=None,
         no_delete_temp=False,
         storage_options=None,
         no_count=True,
-        max_merge=200,
         chrom1=2,
         pos1=3,
         chrom2=4,
@@ -383,17 +385,18 @@ def test_cload_custom_tempdir(bins_path, pairs_path):
             testcool_path,
             metadata=None,
             assembly="toy",
-            chunksize=10,
             zero_based=False,
             comment_char="#",
             input_copy_status="unique",
             no_symmetric_upper=False,
             field=(),
+            chunksize=10,
+            mergebuf=10,
             temp_dir=temp_dir,
             no_delete_temp=False,
+            max_merge=200,
             storage_options=None,
             no_count=True,
-            max_merge=200,
             chrom1=2,
             pos1=3,
             chrom2=4,
@@ -411,6 +414,8 @@ def test_load_bg2_vs_coo():
         metadata=None,
         assembly="hg19",
         chunksize=int(20e6),
+        mergebuf=int(15e6),
+        max_merge=200,
         field=(),
         count_as_float=False,
         one_based=False,
@@ -458,6 +463,8 @@ def test_load_zero_one_based_bg2():
         metadata=None,
         assembly="toy",
         chunksize=10,
+        mergebuf=10,
+        max_merge=200,
         field=(),
         count_as_float=False,
         comment_char="#",
@@ -507,6 +514,8 @@ def test_load_zero_one_based_coo():
         metadata=None,
         assembly="toy",
         chunksize=10,
+        mergebuf=10,
+        max_merge=200,
         field=(),
         count_as_float=False,
         comment_char="#",
@@ -566,6 +575,7 @@ def test_cload_append_mode():
             metadata=None,
             assembly="hg19",
             chunksize=int(15e6),
+            mergebuf=int(15e6),
             zero_based=False,
             comment_char="#",
             input_copy_status="unique",
@@ -608,6 +618,8 @@ def test_load_append_mode():
             metadata=None,
             assembly="hg19",
             chunksize=int(20e6),
+            mergebuf=int(20e6),
+            max_merge=200,
             field=(),
             count_as_float=False,
             one_based=False,
@@ -647,6 +659,8 @@ def test_load_custom_tempdir():
             metadata=None,
             assembly="hg19",
             chunksize=int(20e6),
+            mergebuf=int(20e6),
+            max_merge=200,
             field=(),
             count_as_float=False,
             one_based=False,