Lift two boxes with the same mass, one small and one large. The small one may feel heavier—even when you know the scale says otherwise.
This is the size–weight illusion. People often judge the smaller of two equal-weight objects as heavier. The sensation is not a simple readout from the muscles: perceived weight is shaped by comparison and by visual information about size or density.
The effect can persist after a person learns that both objects weigh the same. Researchers continue to test which mechanisms best explain it; a recent study found that perceived weight across object pairs reflected both weight and density cues, with their influence changing according to the objects’ weight difference. So the illusion is robust, but its explanation is more nuanced than “the brain expects big things to weigh more.”