If you are bracketing, are the later pictures darker than the first?
If so, it's a simple case of lower numbers in the pixels to make the file.
Black is 0,0,0. White is 255,255,255. Anything inbetween is a mix.
The darker the image is, the less numbers are needed. Data is only read in 0 or 1's, so that can save space.
I'll run a test with my P850 over the weekend.