The best cyclists going up hills are those with the best weight-to-strength ratio, which generally means being thin and strong. But heavier cyclists go faster downhill. And being light does not help much on flat roads.
Interesting article in the NY Times about the fact that body type is a poor predictor of cycling ability (unlike running, where overweight individuals are at a serious physical disadvantage).
I know in triathlons, I've had some very fast heavier gals and guys zip past me in the bike segment.