From what I'm reading, this setting is only a hint used by the Optimizer:
ROWS_PER_BATCH =rows_per_batch
Indicates the approximate number of rows of data in the data file. +

By default, all the data in the data file is sent to the server as a single transaction, and the number of rows in the batch is unknown to the query optimizer.

But that's what the FastLoadMaxInsertCommitSize does, and I have it set to the default (2147483647):
"Some fast load options are stored in specific properties of the OLE DB destination. For example, FastLoadKeepIdentity specifies whether to keep identify values, FastLoadKeepNulls specifies whether to keep null values, and FastLoadMaxInsertCommitSize specifies the number of rows to commit as a batch. Other fast load options are stored in a comma-separated list in the FastLoadOptions property. If the OLE DB destination uses all the fast load options that are stored in FastLoadOptions and listed in the OLE DB Destination Editor dialog box, the value of the property is set to TABLOCK, CHECK_CONSTRAINTS, ROWS_PER_BATCH=1000. The value 1000 indicates that the destination is configured to use batches of 1000 rows. "

So I went and checked FastLoadOptions in the advanced editor and there is nothing in the list, which to me means that this shouldn't happen?