Look up "square-cube law". It governs everything about animal size. But not vehicles, so much.

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.

Does that apply to cold blooded animals as well?

Differently. Actually expending energy produces waste heat, and they don't have mechanisms to speed up shedding it. A big lizard can run only a very short distance before overheating. Galapagos iguanas discovered swimming, and so can be more active than other big lizards.

The only land animals that have perfected long-distance running are humans and pronghorn antelope (which are really a kind of giraffe).

Of course the birds have us beat all to hell, no matter which way you look. Fruit bats would be impressive if not for the competition from birds.

As a body increases in size, the surface area increases as a square but volume increases as a cube. The more volume, the more heat, and after a certain point your available surface area is no longer adequate to vent that heat.

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.