I understand what he's saying, but where I'm at is if you look around the world at other sports you'll see being 6'3 doesn't mean you're not agile.
And that's mainly because from what I can see football treats its players as endurance athletes - and there are very few tall endurance athletes - like marathon runners essentially.
Whether that is the best approach to it is questionable in my book.
The NBA is a perfect example. Arguably the most physically taxing major sport out there, and a large number of the players are both extremely tall and agile.
You'd imagine if the same athletes put in the same amount of training into being a FB, they would still be wildly successful. It's just a matter that the majority of physically gifted athletes don't end up playing FB, it's the least essential position.