If you look at individuals that are natural, there are no cases of extreme difference on muscle mass ammounts.
Lets take the example of people who dont train, if what you claim is true, then we would have non trainees being "massive" compared to other human beings because they had more AR than the rest, but this is not the case.
Also, in true natural bodybuilding competitions, you look at the guys there, and they are all at the same level, if the show is truly tested, there are no extreme deviations from the norm. The extreme deviations only start to happen when there is a possibility of the participants being on some sort of substance, like you see on the "fake natural" competitions.
At mister olympia there is even more deviation, big ramy and winklaar have more mass than the rest, why is that?
Simple: because they take the most ammounts of gear. This sport is like a war, you do everything in your power to win, you take what you gotta take, some are able to withstand taking more, others dont and have to take less, but everyone takes a lot of stuff, nothing compared with the run-of-the-mill cycles we do.
Naturals look all the same because of the natural "ceiling" that keeps them from deviating, thats why after some years of being in the "business" you can tell if someone on the gym is on something or not, because you have an idea of what is possible, and the person in question is starting to deviate from it.