You've just used rare exceptional circumstances as the basis to your argument. That doesn't conclusively prove your point that small heavyweights are going to have as much success against heavyer opponents when there is far more evidence that exists against them to the contrary.
Evander Holyfield is a rare exception. Yes he beat Mike Tyson. However, very few ex cruiserweights can beat Mike Tyson. Evander Holyfield was an exception. Very few cruiserweights since Evander Holyfield have had anywhere near the success as Evander Holyfield.
By consistency, lighter boxers have struggled more often to beat and KO heavier + skilled opponents than vice versa.
Another thing you seem to be mistaking in differentiating between is a heavy boxer that is low skilled and a heavy boxer that is high skilled. Nikolai Valuev is a low skilled heavy boxer. Lennox Lewis and Klitschkos are high skilled heavy boxers. So even if small heavyweights or ex cruiserweights like Evander Holyfield and Joe Frazier can beat heavy but low skilled boxers like Nikolai Valuev, it by no means follows that they can also beat heavy + skilled boxers like Lennox Lewis or the Klitschkos. It's important to differentiate between the two.
Also, Chris Byrd may have been a middleweight once, however, he by no means was one when he turned pro. He was a more natural, bigger heavyweight than Joe Frazier. A boxer's size before they compete professionally is totally irrelevant when judging them as pro boxers.
tyson was a beast (vs limited opposition) with both hands.
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