On the flip side, if you workout and study martial arts, it could allow you to avoid being beaten up by a super star small person. And it could help you fight off a really out of shape big person who might have sat on you otherwise. But if a big person is fit and practices, you will not stand a chance against them, unless you are genetically much faster than them.
But when you see one guy take on 3 guys in movies, that is BS, usually. You'd have to be 3x faster than them and have eyes behind your head to do that. Speed only matters when you are free. Once they crash into you and grab you you don't have speed anymore, and it is just power and weight at that point, and you lose.
Ever notice in the movies that when the small person karate kicks the big person, the big person flies back 20 feet, and the small person just stays right there? That defies Newton's 3rd law of motion: the lighter person should fly further back. That is what is such BS about the chick flick movies. If they wanted to make it look real, they should have her hit him in the neck, so he can lose without flying back across the room. Fact is is 3 guys run at you and you kick them, you might hurt them but their mass will still be coming at you and knock you down. Unless you were fast enough to snap all three of their necks in the spit second before they crashed into you, they will be alive and on top of you and pounding away. Now if you are 120 pounds and can bench press 600 pounds, then you could just through them all off you at the same time. But very few athletes can bench even twice their weight, unless that is the only exercise they do.