Size doesn't Matter
Does it look like the big robot is chasing the little robot? It's an illusion!
The brain interprets a two-dimensional image as a description of three-dimensional space. Therefore, it appears that the upper robot is larger.
Experience teaches us that the further we are from an object, the smaller it appears. Therefore, the brain contains a corrective mechanism that "enlarges" objects we perceive as far away. Here, the mechanism fails because the upper robot is not really further away.
The Ponzo illusion was first demonstrated by the Italian psychologist, Mario Ponzo. In its original form, it consisted of two lines of equal length drawn as railway tracks.
Roger Shepard used the idea to create an amusing picture of monsters, which served as the inspiration for the museum display. The idea also works as a tactile/haptic illusion.
Something to think about and discuss: Why, when you look down from the top of a high tower, do the people below look small, but when you look into the distance on ground level people there don't look small?
A demonstration on Dr. Michael Bach's extensive website on illusions:
Can the Ponzo Illusion explain the largeness of the moon on the horizon?:
A detailed presentation of the Ponzo Illusion and a variety of explanations:
Chairs from Wonderland
Dr. Ames' Enchanted Room