... I have found that the images from my 7D just don't look at nice as the images from my 5Dmk2. ... They just don't look as clean with the same level of detail as the 5D or 1Ds I have. Could be there is some truth to the density of the photosites just becoming too much?
Could you gives some extra details, like which models of 5D and 1Ds you mean, the choices of aperture sizs and shutter speeds involved in your comparisons, and whether you are comparing equal sized prints or crops to an equal number of pixels, as with 100% on screen viewing?
Because if you mean 5D Mk II or 1Ds Mk III, there is a difference of resolution which means be able to get the same display size with higher PPI from those 35mm format cameras, but if you mean earlier models it would be the reverse. Likewise, comparisons at equal ISO speed (and thus with different aperture diameter, diffraction effects and DOF at equal shutter speed) would skew one way, while comparisons at equal effective aperture size (and so different f-stop, and so different ISO speed for equal shutter speed) would skew in a different direction.
But in one respect, the combination of higher resolution (more pixels per mm, and so more lp/mm) with shorter focal lengths and greater degree of enlargement that typically go with the use of a smaller format like the 7D is likely to have the effect of revealing lens and optical limitations more, through that greater degree of enlargement. More so if for some reason comparisons are done at equal f-stop, meaning with smaller aperture size, more diffraction blurring and less OOF blurring with the smaller format.