The perceived size of objects depends on a number of factors. Perhaps the most important of these is the visual angle subtended by the object on the retina. All other things being equal, the object that subtends the larger visual angle will appear larger. If you have already clicked on the visual angle, above, you know that the visual angle is dependent on two factors. 1. The actual size of the object and 2. the distance the object is from the eye.
Another factor effecting perceived size is size constancy. This phenomenon results in objects of known size tending to appear constant in size regardless of their distance. So, for example, if you are looking at your friend and that friend starts walking away from you, the friend does not, at the same time, start to appear smaller even though the visual angle subtended by that friend is getting less and less.
Actually I made a small error in the above statement. Size constancy does not result in perceived size remaining constant regardless of distance. If the distance is large, enough known objects will appear smaller. If you have ever looked at the ground, say from the Empire State Building or the Eiffel Tower or similar very tall edifices, you would have noticed that people on the sidewalks and cars in the streets look very small, indeed. They almost appear as ants.
Another factor that can influence perceived size is perspective. One of the situations where this could be experienced is in a tunnel. The bricks lining the walls of the tunnel will subtend small and smaller visual angles the further they are from your eye. In fact, these converging lines are often used by artists to create the illusion of distance in their paintings. If you have clicked on the word perspective already, you will have noted that if objects of constant size are placed in an environment where there are strong perspective cues these objects can appear larger at greater distances.
One other favorite demonstration is one invented by Adelbert Ames called the Ames distorted room.
Here are a couple of other illusions that yield unusual size perceptions: Muller-Lyer Illusion, Ponzo Illusion
The Ponzo Illusion clearly is related to the effect of perspective noted above. There are several explanations for the Muller-Lyer Illusion including at least one physiological explanation involving later inhibition
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