Here are two optical illusions. In both, all the smaller darker-lined circles are exactly the same size. I'd seen the top one before, and I wondered if it would still work without the center black dot, so I created the second "flower petal" illusion with all four petals as perfectly round as the center.
(The illusion also works when the lines radiating from the center are not perfectly equidistant, since I eyeballed the design quickly using PowerPoint, of all things, as my graphics program.)
I think these work because you assume the radiating lines are literally going backwards into the distance, and you "warp" the small circle into an oval shape because you assume it must be moving backwards as well.