Steve Hoffmann wrote an interesting article with some sample shots discussing DSLR sensor size and pixel density in Canon 1Ds Mk II, Canon 1D Mk II, Canon 20D:
“The 20D sensor’s higher input resolution (higher pixel density) only helps with telephoto photography when your object is to fill the frame with your subject from as far away as possible. If you can zoom in, walk closer or get a longer focal length lens and fill the frame (same FOV), the 1D Mark II is on equal footing and the 1Ds Mark II’s 16.7 mp resolution will rule.
Pixel density, at least in this comparative series of images, doesn’t seem to have had the expected relational effect on random digital noise and dynamic range. The three cameras did equally well with slightly blown highlight reproduction. Even more confusing, the 20D seems to have edged out both the 1D Mark II and the 1Ds Mark II in producing slightly less dark area noise than either of these cameras. The 1D Mark II with its larger photosites should have been the winner here but turned out to be the worse noise producer.”