I thought the shape of the downtube was what mattered the most, not always just the width.[/quote]
actually, both matter a lot.

and the frontal area of the rider also matters a lot, obviously much more than the frame (but that doesn't mean the frame and wheels don't matter at all, because they do). and a rider with a more aero position and/or narrower legs will, all other things being equal, be significantly faster than a rider with a less aero position and/or bigger legs. size does matter.