eh, this is actually not nessesarily true - Ligher cars would very strongly reduce the minium quality of batteries needed to give good preformance, but the supply of electricity is not a limit on how big cars we could use under an all-electric paradigm, because the total size of our electricity infrastructure will be at a minimum big enough to service average daytime comsumption, and there are no current zero-carbon solutions to that problem that do not end up also producing quite ridiculusly large surpluses of electricity at night. If your grid is all-nuclear, the marginal cost of running the reactors at full throttle at night as well as during the day is near zero, and a wind, wave, geothermal grid would by nessesity be overbuilt to a degree that would invariably result in very large amounts of spare electricity at night. which means that if you have a battery that can keep a 3 tonne limo going from dawn till dusk, everyone could drive one with a clean concience as far as fuel consumption goes (less so as far as embodied energy goes.. but then again, if the electricity supply is clean, aluminum bodywork suddenly has fuck-all enviormental footprint.)