Rats engage in "play fighting." Big rats don't beat smaller rats 100% of the time as one might expect. If they did, the smaller rats wouldn't want to play. They beat them ~60% of the time to keep the smaller rats coming back.
Social hierarchies aren't dominated by power, they are dominated by those who are attuned to the nuances of them. They are dominated by those who can win while still get invited to play again.
Those who are hyper-competitive don't get invited back. Neither do those who seem too eager to please.
It is a delicate balance. To win in a social world, you need to have a seat at the table. You need to be able to win while still getting invited back to play.
**This only applies in iterated games. In single games, it is optimal for the large rat to use its power to win every time.