Relative size does matter: Animals see optical illusions.

Abstract

Consider the classic Ebbinghaus illusion in figure 1: the orange circle on the right seems bigger, even though both orange circles have the same size. Such size illusions occur in the real world too: in fact, a common dieting trick is to serve the same food on a smaller plate, which makes it appear larger, so you end up eating less. Why do we get fooled by such size illusions?