+1 for the remaining shot estimate having the potential to be wildly off. I saw the same on my 450D and currently experience this on my (gasp) D700. It reads 605 shots remaining for an empty 16GB card, where I regularly get 1200+ in real conditions.If the camera were amplifying the dark regions more than the light ones, all we would get is a gray image with little or no contrast. Canon has not yet been able to control the gain of individual photosites, and, if they did, they still would not amplify dark ones to brighten them up. Black is black.
Just changing the ISO is not the whole story. File size is a function of the entropy of the image, which in this case is most commonly created through noise. As noise is typically highly amplified in shadow OOF regions, these are the sorts of images that will take up more card space. Images with high ISO shot in good light, such as for high shutter speeds, will not increase the file size as much as images shot in low light at high ISO with large amounts of dark OOF areas.