Two main reasons why: JCC is a walking heavy bag. Never met a punch he didn't like to take. Comes in head first with some of the worst defense in the sport. Can take a shot, sure, but has never been hit by someone with GGG's power. Canelo's stamina is still putrid. GGG walks him down and knocks him out by the mid rounds. These are two huge sticking points for these fighters...and those aren't there only flaws--but the biggest. GGG is simply a more complete fighter. GGG's biggest problem is that sometimes he telegraphs his punches and that's because he puts so much into his power punches. But neither Canelo or JCC would really be able to take advantage of such a flaw.
I mentioned this in another thread, but Jr.'s size, durability and pressure would force Golovkin to fight on the backfoot and on the inside...two things he is rather uncomfortable doing. I think Julio would be a very live dog in this one. Canelo, on the other hand, would probably gas out and get stopped because he'd burn up so much energy trying to evade those Golovkin bombs and get inside his reach to land shots of his own.
GGG would walk both of them down by my estimation. I don't see any scenario where Jr. is walking GGG down. I just can't envision it. Jr's resume is average at best to this point, and some would say that he has more than one loss. GGG would be the best fighter he has fought by a long shot. And need I mention that Jr.'s power is vastly overrated? He has been so much bigger than his opposition yet he isn't exactly going in there and destroying guys. He's a grinder, nothing more.
Of course he'd weigh more. And be slower, and more sluggish, and easier to hit. Size isn't the only factor. Jr. simply isn't as skilled, and that's plainly evident if you watch his fights.
