McGregor is a very big 145, for sure. Great reach, great build, explosive.
Of course they rehydrate to a heavier weight, that's the case in all fighting mediums. I'm not forgetting that.
Diaz weighed in a pound heavier, but at this point I think he's used to being heavier. I think he probably walks around about 5-10 lbs heavier than you suggest, but that's just a guess based upon where he ended up. To my eye, he looked significantly bigger (super scientific eye test).
Like he said in the presser, he came in 'fat, off the beach in Cabo'. Was reading some of his history pre-fight, also just checked wiki, and in 2014 he was removed from the lightweight rankings because of inactivity. He missed making his weight against Dos Anjos in December of that year, ended up at about 160 lbs. They gave him another chance to make the cut, and he declined, didn't even try, and had to give up 20% of his earnings from the fight.
So I don't think it's thaaaat easy for him to cut his weight to that range.
As for talking about McGregor not being able to fight in that 145 range much longer, he doesn't seem to have a problem with the salads.
Do you think cutting weight would have more negative health consequences for him than moving up in class, and taking heavier shots from heavier men? There's a trade off there, and him crushing the competition in featherweight could very likely be less taxing on his body than taking a lot of heavier shots from heavier men.