You have the power to accidentally break someone's neck. That is quite a lot of power, but not really the kind you want to exercise. Or do you? Would you? What happens if you get in a bar fight and actually kill someone? What's the conversation like with the jury?
The truth is that many MMA fighters get into fighting in the first place because they feel so powerless in their daily lives. Many of them come from rough backgrounds, where they had to fight to survive. It was crush or be crushed. So the idea of stepping into a ring and dominating another human being was the only way they knew to feel in control. It's a sad breed of power, but power nevertheless.