[dyna]

Let me give you some example fights of famous KO'ers and how their KO'performance suffered when fighting against heavier opponents:

Vitali Klitschko was outweighed by Lennox Lewis and lost
Vitali Klitschko was outweighed (or nearly outweighed) by Shannon Briggs, Samuel Peter, Timo Hoffmann, Kevin Johnson, Chris Arreola, Ross Puritty, Dereck Chisora and couldn't canvas-KO these opponents
Wladimir Klitschko was outweighed by Ross Puritty and lost
Wladimir Klitschko was outweighed or nearly-outweighed by Sam Peter and Mariusz Wach and couldn't KO them
Mike Tyson was outweighed by Buster Douglas, Lennox Lewis, Kevin McBride, Danny Williams and got KO'ed.
Earnie Shavers KO'ratio against lighter-than-self opponents: 83%, against heavier-or-equal opponents: 66%.