My tires are 700c, I'm assuming that's somewhat standard tire size, it's what came on my bike. And when I first started noticing this trend, I would tuck in and get in a more aerodynamic position once I crested the hill and would go into a harder gear to get as much speed by pedaling...still all those coasting would pass me. It must be a weight thing as my tires are smooth.

I'm certainly not about to try to gain weight to increase my downhill speeds!