Decreasing sail area of the rider can be a factor as well. You'll really notice the difference going from an upright position to the hoods, down in the drops, and even using aerobars. The faster you go the more difference it makes, as drag increases as the speed increases disproportionately.
It takes a little of all the things you mentioned, though, and eventually you get to the point of diminishing returns with light weight, position on the bike and small high pressure tires.
The most important factor is legs!
