Without the fins: four lines of the same length.
Bigger or smaller?
Müller-Lyer illusion
Which lines are longest: the ones with fins pointing out, or the ones with arrowheads?
Every line is the same length.
Why? One idea is that the fins look like the corners of a room (far away) or of a building (close to you), and your brain corrects for distance. People who grow up without many square buildings are fooled less.
Picture: Franz Müller-Lyer, Public domain · Wikipedia(opens in a new tab)