Although it might not look like it at first, both of the filled black circles are the same size, the only difference being the size of the circles around them. This illusion was first documented by the Belgian Mathematician (among other things) Joseph Delboeuf some time between 1887 and 1888.

The reason this illusion exists is because of the way that we perceive relative sizes. While the inner circles have the same dimensions, the black circle on the left appears to be smaller because the circle around it is much larger than itself. Conversely, the black circle on the right appears to be larger because the circle around it is only marginally larger than itself. It is only with concentration and with the setups juxtaposed that one can see that they are indeed the same size.