Pet stores are notorious for not feeding their ball pythons. What does Diablo weigh? It doesn't sound to me like your over feeding at all. Some Bp's will grow faster and bigger than others. Just like people do LOL. It depends on a lot of factors. Sounds like your doing everything right, I would cut back your feedings to every 5 days, and depending on his weight hoppers may be too small for him.

cheers.