A few that may belong in this list in my opinion are
FranK Bruno.
http://www.boxrec.com/boxer_display.php?boxer_id=000505
Bruno was a big, strong muscular guy with real KO power but I heard stories for years that he had a thing for always wanting to come in under 230ibs, something his camp were not too happy about. Around 1992 he started coming in heavier and looked a better fighter for it. Maybe if he had done this a few years earlier he just may have been more effective?
George Foreman.
http://www.boxrec.com/boxer_display.php?boxer_id=000090
I of course mean the 1970's version, Foreman was a real giant, a monster of a fighter. But I am sure like me many of you have all heard the stories that Foreman liked to dehydrate himself before some fights as he was concerned about not coming in too heavy and also wanted to looked 'ripped'. If these stories are true can you imagine was a monster he would have been without these 'weight issues' and was coming in say 10-15lbs heavier and in better shape for it?
Gary Mason.
http://www.boxrec.com/boxer_display.php?boxer_id=003829
For one fight only I felt that Gary Mason at 235lbs was too light against Lennox Lewis. At this weight he seemed to have lost some of his strength and power, his two biggest assets. No matter how light he came in he was never going to be as quick or mobile as the young Lewis. Having said that no matter what weight big Gary had come in that night Lennox would have beaten him.
Any thoughts one the above fight fans? Are there any other Heavyweights you can think of that may have for one or more fights come in too light?