Bigger or stronger won't stop a bullet. Anyone who cheats in a way that seriously damages the other, in a world that has guns in it, is a f*cking idiot.
EDIT: This is known as "deterrence" if the potential malefactor is fairly bright and "evolution in action" if not.
If I barter you my old laptop that crashes when you leave it on more than an hour, you're going to be mad later when you find out I didn't tell you, and you'll have to appeal to a third party for justice if I'm bigger or stronger than you.