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Even discussing Test levels is neither here nor there. These genetics that play a role are largely due to skeletal frame and mostly, to myostatin levels. Myostatin is what stops you from gaining muscle no matter what training, diet, and steroid use allows. More test? Pro BBs have been using for a very long time, and are always on to get to where they are at, so that has nothing to do with it.
Look at Belgian Blue cattle:
Lifts weights? Nope. Eats a high protein diet? Nope. Steroid use? Nope. Myostatin gene mutation? Yes.
Same thing here:
Mouse on left is normal, mouse on right has a myostatin mutation.
Again, this along with a few other factors are the genetic traits that allow continued muscle growth, which is what we mean by "genetics". It has nothing to do with how many calories they need to grow or how much natural testosterone they produce, which is nil when they get to or are near pro ranks.