The Gestalt laws perceptual processes governing. Experience and comparison with the everyday is one of the mechanisms used in the process of interpreting what our senses perceive.
The optical illusions are simply a misunderstanding of mental reconstruction of the outside world caught on view.
One mechanism for identifying the size of the objects is to apply simple rules perspectives, so the closest elements, equal in size to the projection, will be larger than those that are distant.
Another way of saying this would be that two objects of equal size larger if we see are close to the observer and smaller if they are away.
This leads to wrongly perceive the size of two equal elements, as different, considering that measure the same in space but they are at different distances.
The perspective effect of an image can help magnify this phenomenon.
What else helps create this effect? Can we increase it?
Another factor involved in this illusion is the distance between the elements to compare. If you are close the effect is less to power mentally superimpose objects more easily (After the measure is moving and compare).
See -> Percepción