[cmyr]

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.