Size Constancy

Size Constancy is when the brain perceives objects as having constant size even as distance changes.

http://vignette1.wikia.nocookie.net/monoculardepthcues/images/2/26/Relative_Size_2.jpg/revision/latest?cb=20100323151151

In the first picture, it seems that the man in the blue is the same size as the man in the red. The second picture shows how size constancy can trick our brains into thinking that the two men were the same size.