Gallileo proved pretty conclusively that descending has little to do with weight. Coasting speed downhill is about terminal velocity, which is determined by aerodynamics and rolling resistance. Where weight might come into play is that that among us who are a little...ahem....rounder, are likely to be more aerodynamic than skinnier riders who are all angles.
I'm also a fast coaster, and I usually have to feather the brakes to avoid running into my leader, even if he/she's actively pedaling. I'm 180 lb, on a standard non-aero frame. The only things I have going for me are 25mm tires with latex tubes, and medium section Rolf Vector Pro wheels, which have low spoke count (14F/16R) bladed spokes and phenomenally good bearings - when I spin one of the wheels on a stand, they go for minutes.