Steinbeck's novel is set during the American depression, a time in which ranching became the crucial way of life for a large proportion of the population. At the time, the ranch owner - "the Boss", held a huge amount of power that shaped the lives of the men who worked for him. He provided accommodation, paid wages and offered an alternative to the completely bleak and lonely existence that these mostly single men, without a family and without any other companion, would otherwise face. It could be said that he had the power of life or death over these men.
Curley, being the Boss' son, understood that he had an authority over the other men that allowed him to be the "mean little bastard" that he was. "He hates big guys", Candy tells George after Curley tries to "take after Lennie". His reputation in the ring makes him overly confident and turns him into a bully that sees everyone as a potential opponent. He picks on guys bigger than him to fill some lonely pit of insecurity within himself that wants the world against which he has a grudge to know that he is a "big man" despite his appearance.