I am a bit confused as I am considering to upgrade my canon 350D to 600D. The pixel size has reduced from 6.4 microns to 4.3 microns, however, top ISO has increased from 1600 to 6400. How did that happen? Shouldn't larger pixel have lesser noise? At which step/process did the noise actually reduce? The sensors should be shot noise limited and not read out limited.
Further, diffraction should also start having its influence at larger aperture with smaller pixel. Isn't it?
What is going on?
I would be glad to have a detailed technical explanation, if that can clarify my doubts.