True, McClellan's much bigger than 175. Come fight night he weighed close to 190 pounds, so he outweighs both Nardico and LaMotta by a fair margin.
Lamotta was in the 4th last fight of a career that featured over 100 fights. It's extremely safe to say he was well short of his best at this point. His chin wasn't cracked either, it was a corner stoppage between rounds. LaMotta was actually dropped with plenty of time left in the round and still hung on under plenty of pressure until the bell rang and walked to his corner unaided. I think there's a bit of possible myth around regarding McClellan's in ring weight. No doubt he put on plenty after the weigh in but near 190 is a stretch for me. There's quite a bit of contrasting information out there and not too much conclusive proof unless something has been missed.
McClellan is not KOing Lamotta. Gerald was certainly big, hit way harder than Jake & was a good boxer. Lamotta was a better boxer with a better chin and was a tougher, more clever fighter. If one just looks at numbers, weights & measurements, then Jake appears to be at a disadvantage. BUT he fought way more often against way better fighters and this is a huge part of the reason why he was so much better.