Conor isn't carrying "extra weight". He is basically fighting what he re-hydrates up to. Conor puts on a lot of weight before fighting at 145, upwards of 15 pounds, maybe more. It is why he looked like a skeleton and why many speculate he might not actually be able to get back down to the weight. You could have used this argument for the first fight (he just ate a lot), but not this fight.
Jonny Hendricks is the perfect example. He has always killed himself trying to get down to 170. It has got to a tipping point where he can't do it anymore, and he still tries. And as he does, he looks like a shell of his former self. One of the most famous examples is of course Roy Jones, who after putting on the muscle to fight at HW, couldn't get back down to LHW without losing himself.
And heck we can go farther on this very card and look at Rumble, who was looking worse and less physically able at 170 and 185 then he is at 205.