Curley is the son of the ranch boss, so he's got a big head—which doesn't quite matchup with his body. He used to be a lightweight fighter, and he just can't let go of the life: he picks fights just to prove himself on the ranch among other bigger (and better) men.
Candy lays it out for us:
Well . . . tell you what. Curley's like a lot of little guys. He hates big guys. He's alla timepicking scraps with big guys. Kind of like he's mad at 'em because he ain't a big guy. You seen little guys like that, ain't you? Always scrappy? (2.91)
Stereotypes aside, what we see here is another character who—like Lennie, Curley's wife, or Crooks—is defined by appearances. Lennie is big and therefore dumb; Curley's wife is a woman and therefore untrustworthy; Crooks is black and therefore inferior; Curley…