He hasn't been lifting long and "GH guts" are caused from a lot of different things. Massive hormone intake and massive food intake can sum it up.
What Phil is a MASTER of is blowing up too. He's been blowing up from the second he started training. He used his drugs bang on and found out precisely what works for him. Jay told him the first time he saw him that he could be Mr. O as well. With that kind of encouragement and guidance, you know that Phil's going to be cognizant about his stomach and not letting it become distended.
