In my high school biology class, the teacher asked what would happen if humans were twice the size that we are. The answer was that we’d overheat because we would not have enough surface area to dissipate the heat we generate.
I thought of a question at the time, but I didn’t ask it: If humans evolved to be 12 feet tall and proportioned as we are, wouldn’t we have developed some way of dissipating the heat? I understood that if we suddenly doubled our size with no other changes to our systems, heat would be a problem; but if we evolved to be bigger, it seems to me that the heat problem would have been taken care of eons ago.
Put another way, why aren’t we half the size we are? Our heat dissipation systems evolved to handle the heat generated at our present size. One can imagine a race of “halflings” wondering what would happen if they were the size we are, and wouldn’t we overheat?
If we evolved to be 12 feet tall then mother nature would have had to come up with something to allow us to survive. Otherwise we’d be extinct.
Maybe we did start on the path to 12 foot creatures but much beyond 6 feet or so our biology couldn’t manage so greater sizes were selected out during evolution.
Also, there are more ways than built in cooling to permit a biological entity to cool itself. It could be nocturnal (nights are cooler), it could be more sedentary (not move around much so as not to heat up) or it could be restricted to cooler climates.
Assimov wrote about this ages ago - the area of the skin and the volume of the what the skin would hold.
True if the whole evolved together we’d have larger hearts (or a space to handle the head alone), some adaptations in blood vessel size and maybe a second kind of pump to get leg blood back up to the heart (or rest every few minutes, laying flat to ge the blood from the legs back to the heart).
Pygmies have been considered a heat adaptation. Tall thin bodies (cro magnon for example) have been considered a head adaptation and Neanderthals’ thicker bones and bodies a cold adaptation.
The amount of heat produced by the body is approximately proportional to the volume, (proportional to height-cubed if shape remains constant). The amount of heat lost is approximately proportional to the surface area of the body, (proportional to height-squared, if shape remains constant). Doubling height, width, and depth will increase heat production by approximately eight-fold but heat loss by only four-fold. A new equilibrim will be reached at a considerably higher temperature (because heat loss also depends in a complicated way on the temperature difference between the body and environment).
The reason that elephants have such large ears is that they are a heat-dispersal mechanism to compensate for what otherwise would be too much heat-producing volume relative to the heat-losing surface area. Woolly mammoths, on the other hand, had very small ears, to conserve heat in their colder environment.