A part of it was that to make ice engines more efficient, they got more complicated, which made them heavier. Weight and efficiency standards are looser on larger categories of vehicle, and consumers typically like more space and perceive them as safer.
So you sell more of them and it's easier to keep pace with the regulations.

My last vehicle switch was from a hatchback to a compact SUV. New car weighs twice as much as the old one, but also gets significantly higher fuel efficiency.