I buy it, he's looking pretty big and lean in that muscle up vid.
Apparently he bulked up after the Alves fight. Said he felt weaker than he should have. That said, despite all that, he's still not in the 'huge welterweight' category reserved for guys like Hardy, AJ and Alves (of two fights ago).
About his weight cutting method, Jon Chaimberg has an interview where he touches on it briefly. He claims Georges doesn't sit in a sauna at all. Apparently he carb/sodium loads a week out for a couple of days and depletes 2-3 days out from the morning of the weigh in - or something along those lines, I don't recall the numbers very well, lol. Chaimberg also mentions that the method is pretty standard stuff, so I'm sure the info is out there.