Rumble has the ability to bulk a huge amount, but he's not the biggest, no way in hell.
The guy made 170 a couple of times, and while he looked dead at the weigh ins he didn't look scrawny at all inside the cage, now tell me Bones, Gus, OSP, Bonnar, Griffin, Glover or Gian Villante could ever make 170, their skeletons alone probably weigh more than that.