People have got to stop using this argument. It's absolutely too simplistic. They may eat only grasses (gorrilas get meat proteins from insects, let's not forget that) but they're that muscular because:
A) their bodies are wired by nature to reach a certain size naturally. This is accomplished because
B) their metabolism is designed so that the majority of their nutrients, especially proteins, goes to mainly muscle building so that they can maintain their natural blueprint to live as efficient a life within their environmental niche as possible.
With humans, the majority of our proteins go to facilitating our brain, thus why it's so highly complex and the most intricate of all animals. Remember it was our predecessors' varied diet, including a lot of meat consumption, that helped the humanoid brain get larger & larger so that we could eventually develop the general intellects we have now. This also explains why we're the worse physical specimen of all the animals on Earth. For us, brain nutrition >>>> eating to get brolic & faster.
So saying "this and that animal are vegetarians, look how big they get" doesn't prove a thing. It's about how that animal's metabolism parses out those ingested nutrients that matter. A lynx may eat more proteins than a cow in an average meal, but it's DNA will always limit it's growth potential so that it doesn't blow up to a cow like size.
Let's also not forget these animals do calisthenics & body weight exercises virtually all day. It's total apples vs. oranges.
He probably gives his kids CORN flakes 
