One outline, copied three times, fits each cross exactly.
Bigger or smaller?
Sarcone’s crosses
Which blue cross is the biggest?
None: all three are exactly the same size, though the one between the big squares looks biggest.
Why? Like the Ebbinghaus circles, your brain judges each cross by what is around it. Gianni A. Sarcone drew it with squares instead of circles.
Picture: Ruotailfoglio, after Gianni A. Sarcone, CC BY-SA 4.0 · Wikipedia(opens in a new tab)