I think aggressive kids become stronger adults because the same thing that made them aggressive when they were young is still with them, and if you’re aggressive you’re going to prioritize your muscles because “might makes right” and go to the gym more often and such.
IN short, these are humans and they make choices that are logical to them.
So a genetic underlying cause? Perhaps. Testosterone? Perhaps. But more likely, it’s _whatever_ it is that caused the extra aggressive behavior when they were young, sociological, psychological, chemical, etc.
Then they chose to be stronger as adults.
I wasn’t a bully but I’m very strong. I had powerful leg muscles when I was little, which made me run REALLY really fast, but wanted upper body strength. So, as a teenager, I did exercises. [muscle resistance (Charles Atlas), then gym when I was old enough. Did more of it later on].