As a guy who has played a little, subsequently got into a bit of bodybuilding, and has remained a football fan for about 60 years, I've finally gotten a better look at Barkley's physique, and he's amazing and unusual.
He's got proportionately shorter and a very thick and powerful legs. Both thighs AND calves. He's got thick joints, both ankles and knees. A build like this is ideal for a RB from a number of perspectives. His lowers are very strong and powerful. He's less likely to be fragile and get injured, or injure himself, in part because the structures are more robust, including the connective tissue. And he'll have a lower CG, both because of his leg proportions and because he carries a fair percentage of his weight below the waist. This also tends to offer significant advantages for balance. And that's a natural advantage for a RB, the opposite of someone like Brandon Jacobs who, while enormously strong, was so tall, and with long legs that tackle attempts below or at the knee were his Kryptonite.
But a build like this can also have natural disadvantages. Often thicker joints can come with less agility, and thick-legged guys also don't usually have top notch speed compared to other RB's who are built more like Usain Bolt, who has a build optimized for speed.
But Saquon along with the advantages inherent to his build IS very fast, IS explosive and IS tremendously agile.
The gifts he has in combination, if not unique, are extremely rare. You just don't see many guys built like this, and those you do, can't do the kind of stuff he can do.