This guy is over complicating the simple rule.

Big, lean, natty, choose two. That's it. If you look at someone, it's pretty easy to tell 99% of the time.

First, we have to establish what "lean" means and what "big" means in this example. I'd say lean means sub 10% and big means 18+ inch, arms 46+ inch chest, 28+ inch quads territory.

You can have 18 inch arms, a big chest, big legs, big everything really, and be 100% natural. You'll just be fat. No, you don't have to be "fat" in real terms, but in bodybuilding terms. You'll be at least 15+%. I can personally attest to this being 225 at 5'11 with 17 inch arms, at 18%. I've always been natty, never been under 15% in my life because of it. The reason? If your natty you just have to eat too much food to be big for you to also be lean.

Now, you can also be sub 10% and be natty, for sure. You can be 8% bodyfat and shredded up. But if you're 6'0, 8%, and natty, you won't be big...you'll probably be 160-170 pounds with 16 inch arms at best. The reason? If you're natty you just have to eat too little to be lean for you to also be big.

If you're big, have 18 inch arms, a big chest, big capped shoulders, and you're sub 10%, you're on gear.

Being a genetic freak really has more to do with insertions, fat placement, height, etc. It doesn't mean you can just be huge and lean at the same time, that's just pretty much impossible for dietary reasons.

It's really that simple.