Pacquiao -De La Hoya comes to mind pacquiao might of weighed the same or more on fight night though jones jr and ruiz
Pacquiao vs Margarito where Margarito had 17 pounds advantage with the height and reach in that fight.
Size isn't all at all. But yeah a good match that most thought was gonna go just one way was when Sugar Shane Mosley fought Antonio Margarito. Everybody thought Margarito would have an easy time but no Shane exposed the crap out of him and basically ended his career.
It depends whether you're talking about height or weight or some combination of both. I'm pretty sure the guys above are correct about Haye-Valuev being the biggest disparity in weight. The biggest in height is probably Chagaev (5'11) vs Valuev (7'0).
I would rank that up there but Margarito got caught cheating before that fight so it kinda takes away from the win. Yea he still weighed more and had height but the power was a no show