The small photosite myth
I see you are another who has fallen for the small photositer myth which is repeated as receivedc wisdom everywhere. The important issue is not the size of the photosites (within reason), but the total sensor area as that, with suitably scaled lenses, defines the amount of light available which is ultimately what limits the image quality.
The Dynamic Range of small photosites is lower than that for larger ones as the noise floor is ultimately limited by photon shot noise. However, that ignores the increased spatial frequency of the smaller photosites. Feed that into the mathematics, and for any given output size and common technology, the noise and DR effects are cancelled out by the increased spatial frequency. Further, as on all but the Foveon-sensored SD1, sensors use bayer-pattern 2x2 colour mosaics, the colour fidelity of an image will continue to improve until the 2x2 cell size drops below the resolving power of lenses (which would need about 100MP on an APS-C sensor). Of course the (not much over 1 micron) photosites on some camera phones have long passed the abilities of their lenses to resolve detail (and they also tend to be diffraction limited), but the real damage is simply the sensor is very small and there's limited light to deal with.
There is also plenty of evidence that increased MP counts on systems cameras are not damaging image quality - quite the reverse. If you go to the DXOmark site and look at the overall sensor ratings of any one manufacturer's sensors of the same size, there is an upward trend with MP counts. Of course technology changes help too, but even on common-technology sensors, the higher MP one tends to rate higher at common technology levels.
Sensors can be optimised for different purposes. Nikon's FF D3S gets better high ISO than the much higher (but also older) D3X, but its optimal range image quality is worse. Hence the older, higher-MP D3X has a higher sensor rating than the D3S albeit the former is based on a Sony sensor design now a couple of generations old.
For an informed thread on this (with links)