Saying that they'd likely be more powerful on average than the '45 ones is a huge understatement. Fat Man was 20 kt, every Trident II missile can carry 12 475 kt warheads. Also, if you distribute nuclear explosions across the world, it's much different than having one meganuke in terms of its effect. Meganukes are very inefficient with their power due to the diameter of an explosive sphere relating to the cubic root of the power. I think the original commenter is way off in their proclamation that we couldn't nuke ourselves to extinction.