A copy of A slides onto B: they match exactly.
Bigger or smaller?
Jastrow illusion
Which shape is bigger, A or B?
They are identical. Cut them out and lay one on the other to check.
Why? Your brain compares the short inner edge of A with the long outer edge of B, which sit right next to each other.
Picture: Drawn by Science Playground, after Fibonacci · Wikipedia(opens in a new tab)