between two top level fighters or at least between two top 20 ranked heavyweights in the world at the time. Usyk beating 282 lb Fury while weighing 226 himself. A 56 lb difference. Hunter beating 256 lb Bakole while weighing 213 lbs himself. A 43 lb difference. Usyk beating 262 lb Fury while weighing 223 himself. A 39 lb difference.
Joe Louis at 207 lbs beating 242 lb Budy Baer. Not the biggest difference but 35 lbs is a significant difference.
Not quite what the OP asked for as one wasn't ranked at HW and the other wasn't world class, but 150lbs Charley Burley giving up 70lbs in dominating 220lbs Jay Turner is worthy of mention.
James Woody (198 lbs) vs. James Beattie (234 lbs) Gunboat Smith (180 lbs) vs. Jess Willard 230 lbs) Kid Norfolk (182 lbs) vs. Bill Tate (235 lbs) Gunnar Barlund (200 lbs) vs. Buddy Baer 242 lbs) Rusty Payne (184lbs) vs. Sid Peaks (217 lbs) Levi Billups (211 lbs) vs. Mike White (270 lbs) Tommy Gibbons (179 lbs) vs. Tiny Jim Herman (225 lbs) Mike Weaver (207 lbs) vs. John Tate (232 lbs) Chris Byrd (211 lbs) vs. Vitali Klitschko (244 lbs)
Jack Dempsey-Jess Willard-Toledo Ohio...July 4, 1919...Jack...6 '1...187 pounds...Jess...6'6...245 pounds...a difference of 58 pounds...
This is the right answer imho, although the difference in the fight Haye Vs. Valuev was greater than 58lbs.