Each camera may have different pixel size. Let say Canon 550d sensor size is 22.3mm on 14.9mm. Highest resolution is width: 5184 pixels, height: 3456 pixels. 22.3 divide 5184= 0.0043mm/pixel=4.3 microns/pixel Comparing to Canon eos 5d mark 3; sensor size is 36mm on 24mm. max resolution 5760x 3840. 36/5760=0.0062mm/pixel=6.2microns/pixel. Larger pixel collect more light, and our image have less noise, looks better. Pixel size answer the question: why the resolution is not so important to get better quality pictures and why pro series DSLR cameras cost so expensive.