The correlation between larger vehicles and profit margins is mostly because larger vehicles also cost more to operate, and then people with less money tend to avoid vehicles they can't afford to put gas in, so the people who buy larger vehicles usually have more money. In other words, nearly all large vehicles are premium vehicles whereas some small vehicles are the economy segment.

But that wouldn't stop them from making high margin vehicles that are low to the ground. The automaker with the highest margins is Ferrari.

The real problem is the nonsensical CAFE rules. If you want to make a high production volume small car with premium performance, you're going to blow your fuel economy target for that vehicle class, so they're de facto banned from making those. But >300 horsepower mid-sized SUVs? No problem. So premium performance cars are banned but premium performance "trucks" are fine, what are the premium buyers going to buy?

The companies are just making the thing people are buying. Pass stupid laws, get stupid cars.