Why can black clothes look slimmer but feel hotter in sunlight?

Answer: Absorbed light becomes heat

Absorbed light becomes heat ✓ — Right. Physics sources agree that black objects absorb much visible light and that absorbed light energy can become heat. Clothing-perception sources separately report that darker or lower-luminance clothing can look slightly slimmer. The same surface property therefore points in two directions: less reflected light helps the visual cue, while more absorbed light warms the cloth. That is the tradeoff hidden inside the black-clothes rule.

Reflected light heats cloth — Close in theme, wrong direction. Reflection sends light away from the fabric; absorption is what leaves energy in the material. The heat side of the question is supported by physics sources on absorption and heat conversion. The slimming side is separately supported by clothing-perception studies on darkness or lower luminance. The shared word is light, but reflected and absorbed light do different jobs.

Black traps extra air — Black can feel hotter, but the cited color effect is not about air trapping. Air trapping would depend on weave, thickness, or fit, which are not the variables in this question. The supported mechanism is optical absorption turning incoming light energy into heat. The visual side is a separate lower-luminance perception effect.