[quantumet]

Photon shot noise (http://en.wikipedia.org/wiki/Shot_noise#Optics) is as fundamental as diffraction - it's a property of light, not a property of the sensor.

If your sensor counts a mean of 100 photons per pixel, then you'll see shot noise with standard deviation of 10 photons in each of those, for a signal-to-noise ratio of 10. If you quadruple your pixel size and now measure 400 photons per pixel, then your SNR goes up to 20.

This is why bigger sensors (more captured photons for same light level and exposure time) are fundamentally better at image capture.

[hahainternet]

Ok this is getting pretty convoluted. I didn't mean to imply that photon noise was not a real thing, just that diffraction is a more direct concern. Photon noise can be amortised by a number of different processes which aren't applicable to the case I was trying illustrate. You can't amortise diffraction errors by longer exposure or better photon capture. Pixel size is a primary concern regardless of its noise characteristics.

[Gravityloss]

Think about shooting with a shotgun at a target. If you have a thousand pellets, the distribution of pellets at the target will be quite smooth. Now, shoot through a narrow aperture that takes out 99% of the pellets. Now you have ten pellets. The distribution can not be smooth. The sampling frequency (pixel size) at the target does not matter much. It is the aperture size.

I realize that is a very flawed analogy but it's the quickest I could come up with.

Here's more: http://en.wikipedia.org/wiki/Shot_noise#Optics