As far as I am aware, current fuel economy standards are primarily determined by the size of the wheel base. Some years ago, the EPA went from a reasonably managed chart to a specific formula that gets a little extreme on the ends.
So you end up with craziness like a 95 ranger required to have 60mpg to meet the standard, and a 2024 f35 super mega ultra cab long bed to have like 3mpg to meet standards. (Numbers are made up, but that is the main idea as I understand it)