Why does wearing dark clothing sometimes make people look thinner?

Answer: Our eyes make bright shapes look slightly bigger than dark ones of the same size, so a dark outline reads as slimmer

Dark fabric physically compresses the body into a narrower shape — Wrong. Fabric does not squeeze a body; a loose black dress is as roomy as a loose white one. The effect is in the viewer's perception, not in the wearer's shape, and it works just as well on a mannequin.

Our eyes make bright shapes look slightly bigger than dark ones of the same size, so a dark outline reads as slimmer ✓ — Correct! It is a quirk of vision called the irradiation illusion: a bright patch looks a little larger than a dark patch of exactly the same size, because the brain's response to light regions spreads slightly beyond their edges. Galileo noticed it with planets, and it is why white text on black looks bolder than black on white. A dark outfit also throws fewer highlights, so its edges blend with shadow and give the eye less width to measure.

Dark colours trick cameras into distorting the figure — Wrong. A camera records the light that reaches it without caring about colour; dark cloth simply reflects less. The slimming impression appears in direct viewing too, so it is a feature of human vision, not of lenses or sensors.