Equal white and black dots can look unequal. Which bias explains the mismatch?

Answer: Bright areas expand

Bright areas expand ✓ — Right. Equal bright and dark marks can look unequal because bright regions can be perceptually expanded relative to dark ones. Visual research links this to asymmetry between ON responses to light increments and OFF responses to dark decrements. The useful surprise is that the eye's size judgment starts before fashion, taste, or conscious interpretation. A tiny dot illusion is the low-level cousin of bigger apparent-size effects.

Dark areas expand — Almost, but the evidence points the other way: dark details are often represented more faithfully than light details. ON responses to light can saturate, while OFF responses to dark stimuli stay more linear in the cited visual studies. That makes dark expansion the wrong mental model. The mismatch is a luminance-polarity bias, not a dark-object swelling effect.

Equal areas stay equal — A ruler would say equal, but perception can disagree with the ruler. The cited vision papers show equal-size light and dark stimuli can be represented differently. That is why the correct answer is not simple physical equality. The interesting part is that the bias begins in early visual processing, not in a later opinion about which dot should be bigger.