A powerful athlete is not necessarily a strong athlete, but one who can exert his strength quickly. If Power = Force X Speed, fighters have to learns to make faster movements, increasing his power. A smaller person who can swing faster may hit as hard as a heavier person who swings slowly.
The fighter who is building muscles should be very sure to work adequately on speed and flexibility at the same time. When you combine speed, flexibility and endurance, high levels of strength lead to excellence in sports.