The best aero tuck of professional cyclists still yields a constant 6 pounds of drag while riding in calm air. A more upright MTB is closer to 10 pounds. He has served as a consultant for USA Cycling's Project '96. Jim has used a mathematical model for cycling in his studies of aerodynamic drag. One of the interesting tidbits in Jim's data is that body size does play a part in drag calculations as you would expect.
Larger riders suffer more drag.
Jim found that this did not tend to handicap them because they also had more power, the power increased with body size faster than drag did. This was due solely to body size. This may be comparable to the increase in drag due to bulky clothing. It may well be that a 125 pound rider bulked out in winter gear now suffers the drag penalty of a 200 pound rider, without the benefit of the additional power. I suspect it is less than that, because many winter cyclists wear winter tights so that their lower half is still about as aero as in summer, except for the larger boots flailing through the air.