Deontay Wilder almost beat an opponent double his size when he stopped 398lb Dustin Nichols in 1 round weighing at 215lb. Has any fighter in history ever beat someone double their weight?
Not exactly a real fight, but Oscar de la Hoya outpointed Shaquille O'Neal who was well over twice his size.
Bob Fitzsimmons 160 vs Millard Zender 300 is the best one i can think of. Problem is that heavyweights really dont get bigger than 260 unless they're lethargic bums, and even then the ones above 230 arent usually that good. 1.5x BW is common for the oldschool HW division
Barbados Joe Walcott nominally fought a draw against Fred Russel, who has been reported as 6' 4'' and 6' 6'', and various weights from 200 to 255 lbs. Walcott seems to have dominated Russel, and at one point even jumped a foot in the air, to land a punch that dropped Russel! Russel was not world class, but he had a win over Hank Griffin, and could probably be described as "a fringe contender."
Buckhorse, who fought around the Broughton era was said to have beat people twice his size, but according to Pugilistica his size was understated. IIRC he was 8 stone about beat people around 14 stone, but I may be misremembering
