I think it will be pretty uncommon to see the 75mb files... There would have to be lots of super bright colours and huge noise reduction settings registered in the camera.
I'm expecting you'd see maybe 45-55 range on most shots (not like that's a small file or anything). I could be wrong though.
On 21 megapixels, I'll only see 30mb on a very rare ocasion, usually 27-28