The small image on the main page is a 256x256 version of a full-size... 4096x4096 image. So each pixel in the small version is going to actually be an averagin

The small image on the main page is a 256x256 version of a full-size... 4096x4096 image. So each pixel in the small version is going to actually be an averaging of a 16x16 block of original pixels. Because they're randomly distributed, mostly that averaging is going to result in medium gray. Zooming out is going to do something similar (though the exact effect would depend on how the zoom algorithm works). Plus, your eyes/brain are going to do some smoothing/averaging of their own even where the screen is able to display individual 1:1 source pixels (consider for example that every pixel on what's probably an LCD you're viewing the image on is itself some combination of red, green and blue subpixels).