In a combination of drugs that does it. How it gets from point A to point B is the unknown. Of course Olympians are very well known for taking it to reach peak human potential but it doesn't mean others who aren't competing aren't injecting themselves with this chit. In fact one of the first questions on those steroids forums when taking HGH/IFG is usually " WILL I GET A BIG GUT IF I TAKE 1 CYCLE". Then you have a bunch of people with random beliefs of how much you should cycle to minimize it's side effects.
People who take PEDS come in all shapes and sizes. Weight isn't a determination. The issue with HGH is that it's expensive so most people won't have access to it. I know a few people who take HGH/IFG as hobbyist lifters that are 200lbs.
But again, it's believed that a combination of drugs, the length of time taking it, the amount, the number of cycles and how your body responds to it that may increase the like hood of looking like a turtle shell.