Because vehicle weight is a major factor in fuel efficiency that’s inherent to the problem and can’t be regulated away. Meanwhile, some people actually need large trucks.
Not sure if it’s the intent but a large vehicle carrying a trailer with 10,000lbs. Is much more efficient than a small vehicle that might need to make 5 or 10 trips to move the same load. In that sense, the larger vehicle is more fuel efficient.
I think it's something like, a plane carrying 400 people over a long distance is more efficient than if those 400 people drove that same distance in 400 cars. Something like that.