It's not that simple. It may appear that way in some cases, but really it's more about body size and torque, stamina, pure meanness and aggression in some cases, intimidation in others, and, ultimately, who wins out in the end. All that is in a serious fight, which in some species is likely to end up with one fish dead or at least torn up and damaged. A fight isn't always over when it's over, either. I've seen apparent losers doggedly reengage over a period of days or longer until the apparent winner is eventually worn down or intimidated and it swings the other way.
On the other hand, it can be courting behavior in some species, a test of fitness and not about winning and losing at all. Or I've had two large and relatively equal males of a species repeatedly get into spectacular tussles and each time disengage after testing each other's strength with no clear advantage and neither one up for a fight to the death.