Why can equal-size white and black dots look unequal to the human eye?

Answer: Bright regions are perceptually stretched relative to dark regions during early visual processing.

The eyes register equal sizes accurately, so any mismatch comes only from conscious guessing. — This gives early vision too much accuracy: equal physical size on the retina does not guarantee equal perceived size. Early visual processing already treats bright and dark regions differently, so the mismatch is a perceptual bias rather than only a later conscious guess. It makes the eye's measurement step seem simpler than it is.

Bright regions are perceptually stretched relative to dark regions during early visual processing. ✓ — Correct. The visual system does not treat bright and dark areas in the same way, and bright regions can be perceptually expanded relative to dark ones. That is why white and black dots of equal physical size can look different in size. The bias begins in early visual processing, before conscious interpretation.

Dark regions are perceptually stretched because the visual system overcorrects for shadows. — This reverses the direction of the asymmetry: dark details are often represented more faithfully than light details, and light responses can saturate while dark responses stay more linear. The visual system is not simply overcorrecting for shadows in a way that enlarges dark dots. So dark expansion sounds plausible, but it is the wrong mechanism here.