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There are 2 major things going on here
1. Man can't build small things very well (the bigger it is the better and more efficient it can be made because of the extra room to fudge with) Best example is comparing a big rig to a train, the train is also much more fuel efficient. Very large boats strangely enough are fuel efficient but many times have energy recovery systems onboard. (you usually wouldn't have room in the average car to adequately impliment these things)
2. The motor is sized just big enough for the load in a big rig. If a car would accelerate like a big rig and have a motor HP scaled similar to a big rig (while keeping the motor well built and highly efficient) you would indeed get the mileage you should considering of coarse frictional losses which do not scale. Just look at old issues of mother earth news where folks power their car off a lawnmower engine, go slow and get a lot better mileage. Sizing the motor appropriate to the size of the vehicle reduces pumping losses and puts the motor in the area it belongs. Assuming again the motor can still be made well.
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