He hits pretty hard. Never really commits that hard to throwing monster shots, or landing very long, punch only combos though. He uses his punches to set up the rest of his game, and really, no one can punch as hard as guys can kick, and no punch is as reliable as getting top position and ground and pounding guys out.
Jon just doesn't land a ton of punches on the button. I think it's more of a volume thing. Really, anyone in MMA is strong enough to knock guys unconscious, even without super power, it's more about landing them, and landing lots of them to up those chances.
You guys don't really think some of these KO artists that are way smaller, weaker, less explosive, etc are harder punchers than Jones based on sheer force, do you? lol.