Jon has the upper body of a large heavyweight, but a bizarrely-composed lower body that lets him easily make LHW. He can use his leverage to push around fighters in the clinch and on the ground, and his reach to throw a hugely diverse arsenal on the feet. I definitely think he wouldn't be nearly as dominant at HW, because he does rely a lot on leverage and reach.
That said, he's also extremely talented in aspects beyond size. He's one of those rare fighters who has the mental toughness to never quit, an iron chin, and a superb fight IQ. He is able to adapt freely and has a fantastic spatial understanding.