Emmert's law

Emmert's Law states that objects that generate retinal images of the same size will look different in physical size if they appear to be located at different distances. Specifically, the perceived size of an object increases as its perceived distance from the observer increases. This makes intuitive sense: an object of constant size will project progressively smaller retinal images as its distance from the observer increases. Similarly, if the retinal images of two different objects at different distances are the same, the physical size of the object that's farther away must be larger than the one that is closer.

An easy way to experience this size/distance dependency is by viewing afterimages (which you can achieve by staring at a figure for a prolonged period of time). When we view an afterimage on a distant surface we perceive it as bigger than if we view it on a closer surface, because our brain interprets the image on the distant surface as being farther away.