Younger, discipline in nutrition.
And his not the heaviest out of them.. dude is around 225 lbs give or take a few on fight night.
He comes good and low on fight week in the mid to high 210's then he has a smaller water cut 11-12 lbs then he just starts to eat up, drink up and he ends up with what I said by the time of the fight.
DC for example is heavier, older, his body has done more cuts and harder cuts along the tome (Olympic, wrestling background for many years), fatter.. dude last fight he had was 236 lbs on fight night with just pants on, so yea all that extra fat and age and to heavy it's hard for him compared to Jon's setup.