That rules out guys like Freitas and McClellan, who both entered the ring 3 divisions bigger than the weigh-in. Who stands out for you as a guy with KO power who wasn't milking the weigh-in rules?
Apart from heavyweights, of course, just to back to the same-day weigh-in days: Pipino Cuevas Sandy Saddler Archie Moore Julian Jackson (maybe by the time he was fighting at middleweight we were beginning to have day-before weigh-in and weight bullies like Gerald McClellan were starting to become a thing) Michael Spinks Little Red Lopez Ray Robinson Etc Etc
Rocky Graziano was pretty small at middleweight yet packed a dynamite punch. Rodrigo Valdez was a small middle. I'd be happy to be corrected but i never considered Pipino Cuevas "huge" at welterweight. My god can he bang. Edwin Rosario wasn't a huge lightweight but might be the hardest hitting 135 ever. Two fighters that don't fit but i feel like talking about are Hagler and Burley. Hagler wasn't a big middleweight. Now he doesn't for me fit "KO artist" because reading between the lines of the request it is about huge punchers. Hagler by no means was a huge puncher but he definitely had good to very good power and stopped a lot of guys. He just doesn't have the one punch power for me to throw him in here proper. Burley is interesting. His KO ratio isn't overly impressive but he was matched tough, often fighting guys bigger (sometimes much bigger) and fighting oh so often. I also suspect he cruised a lot in second gear, particularly against lower level guys. Tho he doesn't fit on multiple levels Burley did however drop a lot of quality and durable guys. He had Archie Moore up and down like a yoyo despite being 6 pounds lighter. Guys like Cocoa Kid and Holman Williams found themselves on the mat, Bivins was almost stopped etc. I guess what i am saying is that tho he doesn't fit, in another world he possibly would.
Azumah Nelson started at feather and ended up at super feather. But he carried his power up to that class well.
Pacquiao was almost always the smaller man in the ring. Golovkin was an average sized middleweight, on fight night he typically only gained about ten pounds. Inoue.
Marciano was short and a had a short reach but at 185 lbs was he that much lighter than most of his contemporaries? He fought at a time when most HWs were still below 200 lbs.