fighting has very little to do with muscle size and strength unless all other things are equal.
being trained how to fight, fighting experience, intelligence, speed, toughness all come before muscle size and strength.
I was trained full time for 3 years in Shotokan Karate (that is the real Japanese style Karate), I was one test away from my blackbelt before I got involved in other things. I also trained in full contact and competed in full contact competition, today we refer to this as MMA, back in the 80's that did not exist yet.