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.

(I don't know which one pollutes more).

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.