What I mean is this. Take two cams, one say 3MP and the other 5MP. Both set to auto mode and just shoot the same scene, does the resulting picture on the 5MP cam have a larger file size? If so, is it increasing proportionately to the MP?
Some cameras maintain a fixed amount of JPEG compression for every shot taken at a particular image quality setting so the file size will depend on the complexity of the scene taken. Others like the d70 try to have around the same size for a particular quality setting by varying the level of compression used so it'll be easier to estimate the number of shots that can be taken before the memory card is full.
A shot taken at 5MP with high compression can have a smaller file size than one at 3MP with minimal compression.