I have a Canon 550D with a 16GB SD card. I noticed that, when in P-mode, if I choose an ISO of 100 I can shoot 595 raw files on an empty card. If I choose an ISO of 200 this is 590. And if I choose 400 this number decreases to 583. Here below is the rest of the ISO vs. space
ISO __ | __ Amount of raw files that fit on 16GB
800 __ | __ 569
1600 _ | __ 556
3200 _ | __ 539
6400 _ | __ 510
12800_| __ 477
Does anybody know, why the filesize of one raw photo increases with the ISO?