Define a weight and drop taxes/offer refunds for people who buy them, see more of them appear.
Subsidizing smaller cars is a bad idea because they still have negative externalities, just less so. The government shouldn't be favoring driving a small car over walking, but that's exactly what a small (or electric) car subsidy does.
The reason we subsidize EVs instead of taxing gas-powered cars is because the latter is politically unpopular, not because the former is a better designed policy.
> Consumers who fear for their safety and buy big cars and rest assure nobody else will be driving an even bigger car.
Sounds like a middle-schooler banging out a 3-point persuasive essay asap so he can use the rest of the time to play drug wars on his ti-82.