Going up, the relevant metric is power to weight; on the flats, it's power to frontal area. Coefficient of something or other (the engineers will chime in here, I'm sure). Since big guys punch a fractionally larger hole in the wind with a lot more power, they tend to do well in crits, flat TT's, and flat RR's. Skinny guys tend to better in hilly RR's and hillclimb TT's.

But you knew that.