Both red circles are quite identical. To see this just move the cursor
over the image on the left.
This illusion is based on the shape contrast. Red circles have different
surroundings which influence their appearance. Our eyes and brain compare
each red circle with the circles surrounding it, and in every group
determine the dominant element giving it higher value. In this way the red
circle in the left group is getting higher value than exactly the same red
one in the right group and vice versa. And so their sizes are interpreted
as being different.