Why is it called a semi-trailer truck if the vehicle is very large?

The "semi" refers to the trailer — specifically its absence of a front axle. A semi-trailer is only partially supported by its own rear wheels. The front end rests on the tractor's fifth-wheel coupling rather than on its own axle. Because the trailer is a "semi-trailer," the full combination with the tractor is called a semi-trailer truck. The name describes axle configuration, not vehicle size.

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What is a Semi-Trailer Truck?