Lennie doesn't know his own strength. He also doesn't do anything against Curley until he's been beaten and George tells him to attack. This shows that he's not a violent person, basically, but that he also doesn't understand his own strength. Curley is a bully, which the ranch hands knew. They already knew he was a mean person. Curley is used to getting what he wants and he picks on people, even if they are bigger and stronger than he is. Violence is about power and until Lennie fights back, Curley has the upper hand. Once Lennie fights back, Curley loses his power. Typically, violence is lop-sided. One person (or group) goes against another person (or group) until they HAVE to fight back to save themselves.
Lennie's fight with Curley is very important to the theme of violence in this novel. The fight demonstrates to the ranch hands that Curley is not actually as tough as he appears. Although Lennie does even fight back when Curley initially attack him, once he finally responds, he easily overpowers Curley. Curley has harassed and bullied so many of the ranch hands over the years, and they have all come to fear him because of this. But when Lennie stands up to Curley, it gives a sense of empowerment to the others who quickly realize Curley is not to be feared, at least from a physical standpoint. The other ranch hands also lose the little respect they have for Curley because of the fact that he harrasses and picks a fight with Lennie.
In regard to the description of the fight and it's relationship to the nature of violence in the novel, it is important to note the uncontrollable rage which Lennie exhibits. Although Lennie is not, by nature, a violent person, it becomes brutally obvious that once he loses control of himself, he can be unintentionally violent. Later in the novel, we actually see him become deadly.