I think there can be a rate_scale parameter so the rate calculated can be multiplied by the rate_scale parameter(rate*=rate_scale), therefore user can have better control over "rate" displayed.

For example:
BLOCK_SIZE = 10241024256

before

for chunk in tqdm( split_file( 'largefile.bin', BLOCK_SIZE ), unit = 'chunk' ) :
# do somthing..

after

for chunk in tqdm( split_file( 'largefile.bin', BLOCK_SIZE ), unit = 'bytes', rate_scale = BLOCK_SIZE, unit_scale = True ) :
# do somthing..