38 years old Moore is still better fighter than Canelo can dream off. Moore was in excellent condition in that fight, Rocky simply overwhelmed him. More was also considerably bigger than Canelo and hit much harder. Rocky would walk through Canelo's punches if he did that with Moore.
There's actually not much in it. Canelo is a super middleweight today, but rehydrates to just over 180 in every fight he's had above middle. Moore was obviously very flexible with his weight, but I can't imagine him entering the ring more than five pounds heavier than he was at weigh in time for Rocky. There'd bet no more than 10lbs between them, which isn't that much when you start getting past MW.
Even assuming that Canelo is around 180 lbs during fight, he's only 5'8 with 70" reach. Moore was 3 inches taller and had 5" reach advantage. Moore was also very strong LHW, he was bigger puncher and stronger physically than his weight indicates.
Hopefully the years will be less kind to Mayweather than Marciano. Marciano underrated big time, have any of you haters seen him speak? He was a smart guy not a brainless slugger. 'Ooh he'd compete at middle today he was tiny, like 4'8" he's only unbeaten cos he never fought anyone in their prime' Same posters... 'Mayweather the best ever no one can match him, represent PBF'