It takes more watts to climb, particularly if you are dragging more weight. And you have to sustain it. Good climbers are often light, but the light guys don't put out as many watts overall so they don't do as well on the flats, ie. time trials.

I could pull up a power calculator and demonstate this, but I'll let you play with one instead.

And sprinters seem to come in all sizes. You get that fast-twitch vs. slow-twitch muscle thing going.