First noticed by Mario Ponzo in 1913, size perception depends on the eye's interpretation of depth clues. Two red blocks above are exactly the same size but do not appear as such. The situation reminds one of the Orbison's Illusion and other illusions induced by perspective clues. There is also another illusion atrributed to M. Ponzo.
References
R. N. Shepard, Mind Sights, W. H. Freeman and Company, 1990
