The principle is that 2x size, i.e. 2x length, 2x width, 2x height, gives you 8x volume and, for animals, mass. All that mass has to be kept fed and aerated, and produces heat. But the surface area didn't go up by 8x. It only went up 4x unless you changed the shape a lot. You probably can't get skin to shed heat 2x as well, so you have to be careful not to produce 8x as much heat, instead.

This is all fine up to, say, the size of a giraffe or elephant. But an elephant that runs for five minutes is in for a bad time.

Vehicles are mostly empty space and skin, so they have different sorts of scaling problems.

Another example of a similar principle applies to insects: they don’t have lungs, and absorb oxygen directly through their skin. In earlier eras of earth’s history, there was much more oxygen in the air, and so insects could be much larger, because the same amount of surface area could absorb enough oxygen to fuel a larger number of cells.