Although the theory behind the argument can be confusing, it does have a bearing on which camera to buy. Let me explain by using a mixture of theoretical and practical issues.
Larger pixels collect photons faster than small pixels.
A practical consequence of this is that CCD chips with smaller pixels will take longer to build a usable image. For example, the pixels on the SBIG ST-6 are 23x27 microns. Yet, some advanced amateurs have achieved considerably better results. You might think that, ideally, the smallest stars should be a mere one pixel across. A better answer is three pixels.