It doesnt guarantee anything, but it can help. Yea Ive seen bigger dudes get beat up by smaller guys too. Ive also seen bigger dudes who maybe werent as good a fight as a smaller dude pick them up off their feet and slam them into the ground like a sack of potatoes. Ending the fight in one quick motion.

Most people arent trained fighters. And if 2 untrained fighters are going against each other, the bigger one usually wins.