I don't know if this is true or not, but it sure sounds right to me.
It's similar to when I'm wrestling/play-fighting with my son or, back when we were married, my ex - "beating" them isn't the challenge. Even though I'm a small middle-aged guy (5'5" 160 lb. 44 years), I'm far and away bigger and stronger than them.
The hard part is "winning" without hurting them. It's tougher than it sounds, because they go all-out, while I have to hold back so I don't accidentally hurt them. They can wail away at me all day long, but if don't be careful I could easily hurt them, and then I get, "Why do you have to be so rough? We're only playing!"