Size doesn't equal strength.
Yeah. Size just equals reduced accelerations in case of collision, more buffer area to absorb the accelerations, and in overall reduced shock to the occupants of the vehicle.
It's relatively easy (albeit more expensive than normal car building) to make the chassis strong enough not to cave in on highway speed collisions (example
here). It's a lot more difficult to make sure that the occupants of the car survive. Helmet, HANS and four point safety belts might make it possible, air cushions might help, but with normal safety belts and no neck/head protection, it would be... unlikely to survive, say, crashing to a concrete wall at 110 km/h... and if you get your car under a 12-wheeler you're gonna go splat no matter what kind of carbon-fiber-unobtainium hull with shield projection your car has.