Science would tell you there's no such thing as natural bodyweight vs extra weight, 5lbs of muscle is 5lbs of muscle given the same fibre type. Either people do worse in different weight divisions because the talent is different or the muscle they put on isn't actually muscle and they've trained poorly.

At certain bodyfat percentages you can look like a big muscular guy when really a lot of the mass is actually fat. Its hard to tell fat from muscle when looking at ripped vs shredded differences, but it can still mean a hell of a lot of difference in actual muscle mass. For example I've hovered around 87kgs for the past year, but I've put on half an inch on my bicep and my 5 rep max has now become my 30 rep max. That's a big difference in strength despite being the same weight.

The problem is catchweight's require weight jumps that exceed the fighters capability to put on lean mass given their training regime. Instead they put on mass poorly because they do a cardio heavy regime that is ineffective for putting on muscle quickly in a short time frame.