I have a serious question here: keep in mind that I don't know anything about this sport.
How can a man who looks just a little bit above average be so strong at fighting?
Is it something you master with technique? Is it a natural/muscular gift? Is it a brain/muscle connection?
It's a combination of constant training, conditioning, technique, reflexes, speed and power.
And strength from fighter-specific training, like punching the bag etc.