i wouldn't equate explosiveness exactly with sprinting ability. Explosiveness, as I understand it, is greater acceleration. Sprinters don't have to be explosive in that sense. That's why they have lead-out men. They have to be able to maintain a very high power/speed for a brief time, but they don't have to get up to that speed extremely quickly.Pinot is pretty explosive, whereas Remco can't sprint to save his life.
I didn't say that mass was the only factor. In the case of climbers, it's thought to be that their smaller mass means a greater surface/mass ratio not only externally--handicapping them in TT--but also internally. So relatively more interfaces in the lungs and capillaries that are critical to oxygen transport. More efficient heat loss may also help.Although your conclusion is correct, I have to pick a nit here: power doesn't increase with mass. If it did, climbing wouldn't favor smaller (shorter) riders. Power clearly increases sublinearly with mass.
So power does increase with mass, but not linearly when taking into account internal surface, so I agree with your statement that it's a sublinear increase. That's actually a good way to put it.
Do we know that? How many really tall men have tried to be pro cyclists? Most unusually tall men are likely to be drawn to other sports, where height is a definite advantage. Maybe the problem with internal surface area becomes exacerbated, but I don't think we know for sure that TT ability tends to decrease beyond a certain height. It could be that they are so disadvantaged in climbing that their superiority would not be apparent except on very flat terrain, which of course is not something any racer can expect to find exclusively.And it's not as simple as "taller means better at time trialing", because most of the recent TT world champs are about 6' tall or just over. Even though it's easy to find humans who are substantially taller than that, none of them are very good at TTs.