How do people get their pits to be so stocky and muscular?

Genetically, being very stocky and muscular isn't how the American Pit Bull Terrier should look. They were bred to be lean, strong, quick, agile, and loyal. Most of these APBTs we see today have been crossed with other breeds, inbred, or selectively bred to be grotesquely unlike the APBTs of old.

Being stocky and muscular is a basic characteristic of pit bulls. It's purely genetic, just as it's genetic for spaniels to have floppy ears, or some people to have blue eyes.

Many "pit bulls" are misbred. They aren't supposed to be big & stocky. They are supposed to be small, lean, muscular & agile. The max weight of a Pit Bull is supposed to be around 65 lbs. So it does have to do with breeding, but that breeding is with other breeds.

I have a 3 year old pit that weighs 75lbs and is very muscular and stocky. We don't give him anything to make him that way. His parents were both very very muscular and stocky as well. So I would have to say that it is the way they are bread.