If you specify a batch size during a fast bulk-copy, each new batch must start on a new data page, since only the page allocations, and not the data changes, are logged during a fast bulk-copy. Copying 1000 rows with a batch size of 1 requires 1000 data pages and 1000 allocation records in the transaction log.
If you use a small batch size to help detect errors in the input file, you may want to choose a batch size that corresponds to the numbers of rows that fit on a data page.
