If weight is always worse, then how does the Jacobs Honda (on the EM home page) more than double it's FE with ~33% more weight? How does the Edison2 VLCe more than double the efficiency of the VLC with ~25% more weight?
At lower speeds rolling resistance is the majority of the load. Above a threshold that depends on the CdA of the car, the aerodynamic drag becomes the majority of the drag.
Speed X weight = kinetic energy. So, once you accelerate the weight, you can coast. With lower aero drag, you can coast farther with a given weight.
Aerodynamic drag trumps weight, because aero drag is a total loss, and because at most speeds aero drag swamps all other losses combined.
Edit: I do think that with the significantly lower efficiency of IC engines, that weight is closer to aerodynamic drag; but aero still is more important. The
Spira4u (X-Prize vehicle) weighed about 485 pounds, while the
Edison2 was about 830 pounds, the Edison2 was more efficient.
The Illuminati 7 at 3,100 pounds is about 2X more efficient than either one. When you are wasting 80% of the energy under acceleration, it matters a lot more than if you are wasting only 20%.