Yeah, though not a KO artist per se, Hags did carry respectable power. Focusing in on his size, for all his muscularity, I did read someone’s comment about Hagler appearing fine boned when they met him in person. Also, perhaps due to an illusory angle, but Duran did appear ever so slightly taller than Marv in their pre fight face off. At any rate, making all possible due allowances for same, as he appeared next to Duran, is it possible that we might talking 5’8” to 5’8 1/2” for Hags? Certainly Leonard had him in the height dept. but despite tales of the tape, Leonard’s own true height was about 5’9” to 5’9 1/2” - Ray himself admitting to deliberate inflation of his height during this fighting days.
Again, not necessarily a straight up KO artist but an apparently 5’6 1/2” Qawi might perhaps fit the description here otherwise.
Fitz and Wilder are the obvious ones. Here's some others. With topics like this you are always forgetting someone. Bill Hanrahan is the only champion with a 100% KO rate. He weighed 165 in the Hart win. He is probably the worst champion in boxing history. He had nothing but power. Root had 2 fights at HW. In the first one he knocked Hart down in a thrilling loss in the 2nd one he knocked down a super heavyweight in a win. Root was under the LHW limit for at least the first one. His weight for Russell is unlisted but boxrec notes Root weighed quite a bi more than the 171 he'd weighed the previous fight. This was Root's last fight. So far as abrupt retirements go 29 isn't that young but it's one where he could have done a lot with another 5 years. Ketchel has a 90% KO rate(and I adjusted for NWS) as a MW who made it up to HW at the end. And knocked out Porky Dan Flynn. Langford bulked up later but he won his NSC HW title at 158 pounds and went on his earlier run weighing less than 168. His dominant win over Lang he was 165. Miske got serious power at HW he never demonstrated as a LHW and KO'd Fulton. Braddock started his career as an emaciated 6'2 power punching LHW who weighed under 170 pounds and then as a HW fighting a few pounds over 175. Braddock was not Bob Fitzsimmons powerful and this strategy backfired horribly once Braddock reached the top level but he was powerful enough where someone actually thought this was a good idea. Curtis Sheppard. Sheppard had serious power as a 5'11 guy under 190 pounds. He knocked down the largest HW of his era Lee Murray at least 10 times in their 6 fight series. Murray won 4-2. As a boxer Sheppard wasn't even Marciano level and his best opponents often came back from being knocked down. But Sheppard's power was probably quite a bit harder than Marciano. Shane Mosley. Mosley in another life could have been a neo Langford. At LW Mosley was a monster. At WW and 154 Mosley was in an awkward spot where he created a lot of highlights but didn't have the gas to take out opponents with single shots he wasn't otherwise beating. At LW he could overpower these smaller opponents with his size and at larger weights he would have been the better boxer but at 147/154 he was at the worst place for his skills.