We use a lot of information to judge how big an object is.
One way is by comparing one object to another.
If an object is next to something large, it looks smaller, like the circle on the right.
If an object is next to something smaller than itself, it looks larger.
Thus, the surrounding circles serve as comparisons, and change the perceived sizes of the two center circles.
The take home message is if you want to look taller, stand next to someone shorter.