bro
from what I can tell its a body comp thing!
You body doesn't want to hold a certain amount of muscle without a certain amount of fat. So if you get up to 200 lbs at 7% bodyfat, I believe in order to hold that muscle you might have to have 10% bodyfat naturaly or your body wil fight against holding that muscle.
I terms of the genetic limit thing, I think its how much of that muscle you gain can you keep while keeping body fat around 7-12%. Yo many people are afraid to lose their abs post cycle, well if you want to stay above your natural limit you better ad some fat.
The body will only see a need to maintain those 19 inch arms if you need them. Lifting makes you body "think" it needs them. Now it must "think" that it can support them. With proper diet and bodyfat %.
Of course everybodys bodyfat % set point is different and it can be changed, same with your muscular set point.
And looking at old pros they probaly are 15-20 lbs heavier than if they ever took gear, so maybe gear use over time can raise your muscular set point 15-20lbs. So instead of a genetic limit of 5'10 200 7%, you can have 5'10 220 7%, but foget about maintaining 5'10" 275 4%.
Just my theory from observation and also knowing some steroids(like tren) actually create new muscle cells instead of just enlarging the ones that are there already.