The structure of your individual brain has a lot to do with how you perceive optical illusions. Researchers at University College London asked subjects how they perceived illusions of size such as the one used in this video, and then measured the size of each subject's visual cortex -the amount of brain matter devoted to processing vision.
The researchers then took MRIs of the subjects' brains. What they discovered astonished them - there was an almost perfect link between the size of somebody's visual cortex was and how much the optical illusion affected them. The smaller the visual cortex, the more a person was taken in by the optical illusion. Those with the largest visual cortices were also those most able to see the circles' true sizes.
Read more, and see the different illusions used, at io9. Link -Thanks, Greg Ross!