And yet trucks are huge without a material increase in bed space. If the auto makers can skirt around the law to make everything a truck, surely In this hypothetical, they would make smaller trucks and make them "cars"

You'd pretty much have to repeal a ton of safety laws. Today's Tacoma is the size of an f-150 of 20 years ago, but still can't tow or haul as much. Gotta deliver practicality!