Size-weight illusion

Size-weight illusion is also known as Charpentier illusion (or Charpentier-Koseleff illusion). It is named after French physician Augustin Charpentier. The illusion is manifested in the tendency to underestimate the weight of the bigger object (for example a box) when compared to a similar smaller object of same physical mass, and vice versa. Also, when having two same sized objects of different masses, heavier one would appear smaller. The illusion is caused by integration of visual and proprioceptive information which is not in match with experience.

One explanation of how the size-weight illusion works is that by looking at a larger object, people expect it to be heavier and therefore lift it with a greater impulse. This causes the larger object to lift easier than the smaller object that the viewer thinks will be lighter. This hypothesis has been proven in a test when strings were tied to the bottom of small and large cans and it was found that greater energy was used to lift the large can.