Welcome to the thread.
Actually HGH, from my understanding, is not as effective in stimulating muscle mass in humans. It is great for recovery, injury prevention, and decreasing of body fat. Studies surrounding its capacity to directly influence muscle mass by its own physiological components remain inconclusive.
That is very true, however (and please, correct me if I am wrong) HGH is commonly used in bodybuilding to increase size. They might not be as effective as steroids on a purely muscle building level, but they do still increase size. In Cena's case, the benefit of taking HGH over steroids is that GHG is a protein and therefore can't be detected by the standard piss test that WWE does.