block check sum sizing

[Wallace Matthews]

When I dont specify --block-size but have --write-batch=xxx, I get a xxx.rsync_csum file that is 76 Kbytes in size.
The size of the file varies as the size of the "reference" file is varied. --stats showed matched data that is roughly 6 block lengths
based on the square root of the newer file. 

I copy the original data back to the target directory so that I can repeat the experiment. 
I compute a block size that is the square root of the size of the "reference" file and use --block-size= the computed size.
The xxx.rsync_csum file is 12 K bytes in size. The xxx.rsync_delta file is the size of the newer file and --stats shows 0 for matched data.

I copy the original data back to the target directory. 
I vary the block size to half the previous example. I rerun the experiment. The xxx.rsync_csum file is still 12 K bytes in size. The xxx.rsync_delta file is still the size of the newer file and --stats shows 0 for matched data.

This is non intuitive. If I got 6 matched blocks when the square root is 181,272 then I would expect to get 6 matched blocks when I specify --block-size = 181,272 and 12 when --block-size is 90636. 

I would also expect to see xxx.rsync_csum size to double when I divide the blocksize by 2.

What am I missing??

wally