Distended stomachs arise from a number of factors.
1. Genetic Predisposition.
2. Insulin
3. Over Eating
4. Cutting up, on GH, T3, etc, which allows one to eat a considerable amount of food, and still lose weight.
5. High Doses of GH.... which will cause a substantial enough conversion to IGF-1.
I think it varies slightly between person to person, but the good $$ is on Insulin being the most problematic compound by far.