> I don't think they do optimize volume, because they could certainly have larger trailers.
No they cannot, at least in Europe. The outer dimension of trucks are regulated, such that street planners know what to plan for and truckers know they can pass a street.
People think ya can pop a wind farm anywhere, but ya have to be able to drive the 100m plus single piece blades there, they don't fit on regular trucks, and doesn't always go as planned.
I don't think they do optimize volume, because they could certainly have larger trailers.
The correct metric seems to be diesel consumed per kg/cargo mile.