The left diagonal swings over onto the right one, without changing length.
Bigger or smaller?
Sander illusion
Which diagonal line is longer, the one across the big left part or the one across the small right part?
They are the same length.
Why? Your brain judges each line together with the shape around it, so the line in the bigger shape seems longer.
Picture: CountingPine, Public domain · Wikipedia(opens in a new tab)