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The X-Prize's particular set of rules encouraged extremely lightweight cars. I'll bet the rolling chassis of every single successful car is man-portable. Most made extensive use of composites and aluminium.
In the real world, steel bodies are all but required to keep costs down. Between this and crash safety, you'd be hard pressed to build a rolling chassis that weighs less than 1500lbs.
One of the biggest consequences of an overweight car is the engine has to be sized to your power requirements during acceleration, which means the engine is grossly oversized for cruising where it spends most of its time. That's the biggest reason hybridization works so well in heavier cars. Also, a hybrid system has a harder time pulling its weight when it makes up a higher percentage of the vehicle's weight.
I suppose what you can conclude from the X-Prize is for cars and bikes under 500lbs, don't bother going hybrid.
Here's another interesting factoid: All of the cars that were converted from road car to X-prize car did poorly. None of them met the required mpg(e) criterion.
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