I do not think he is particularly interested in picking on men larger than himself unless he is in a position where they cannot fight back. This is because Curley's problem is that he is a bully.
Bullies tend to be people who do not want equal fights. They want to fight people who really can fight back effectively.
But picking on people bigger than him gives him a lot more satisfaction. He can tell himself that he can beat them up or dominate them because he is tougher than they are. In actuality, it's because he's the boss's son.
So he's a bully who wants to look tough without having to actually have a fair fight.