I make the same power at 145, 185, and 205 pounds; but at 145 pounds I'm about 25% faster up-hill at 145 than 205. People's cardiovascular systems and lactate clearance capabilities are matched to the size they evolved to gathering or killing every thing they eat, not how big around many grow with effectively unlimited access to food.

There's also the issue of genetics. The best climbers are ectomorphs not lugging around excess muscle, with weights generally under about 2 pounds per inch of height (Marco Pantini - 5'8", 126 pounds; Michael Rasmussen - 5'8.5", 130 pounds; Richard Virenque - 5'10", 143 pounds).