Originally Posted by Dane

Heres, the catch. As an object, such as a human body, increases in volume, it has a much less significant increase in surface area. You can add a significant amount of muscle without greatly increasing your surface area, which means that you can make significant increases in power without signifantly increasing air resistance. This means that skinny guys who may be able to power up a climb will be blowing like a leaf (relatively speaking) fighting a headwind while trying to cruise fast on flat ground, because they have a similar surface area as a larger more muscular rider, but less power output.

So to sum it up, speed on climbs depends on a power to weight ratio, while speed on the flats is dependent on a power to surface area ratio. Maybe this will help to give you some insight on how different body types perform differently under varying conditions, and how the stresses on your body change as well.