Why do dark objects look smaller than bright objects?

When any part of the eye is excited by light the effect of the light spreads a little around the edge of the part of the eye on which the light falls. It is as if the light at the edge were radiated sideways. This effect is called irradiation.

When we see a black spot on a piece of white paper, this is what happens: All round the edge of the part of the eye opposite the spot, a little of the effect of the light from the white paper is felt. This makes the black spot appear smaller than it really is. When we see a white spot on a dark background, irradiation works outward from the white spot, and the white spot looks bigger than it really is.

More reading:
Irradiation (Wikipedia)

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