When metal is dented, it stretches thinner. When the dent is pounded out the extra metal needs to go somewhere. That is usually done by heating the center of a depression / peak red hot and letting the metal collapse on it's self.

He might be accomplishing shrinking by leaving the surface slightly dimpled like the surface of a golf ball. In any event, he uses the barest amount of filler.

He is fixing cars with a single pull frame machine that is sub $ 5,000 on the USA used market. Also look at some of the measuring tools he made.