Most of it hinges on the square-cube law, discovered by Galileo, which basically says that your volume (mass) increases a lot faster than your height (and surface area) - cubes versus squared.
It's one of those simple principles of existence that makes so much fall into place once you actually think about it.
To the mouse and any smaller animal [gravity] presents practically no dangers. You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, a horse splashes.
Eventually he addresses why there aren't any sixty-foot-tall people, why insects don't like to get wet, why mice have to eat all the time and why there aren't eagles the size of tigers.
An angel whose muscles developed no more power weight for weight than those of an eagle or a pigeon would require a breast projecting for about four feet to house the muscles engaged in working its wings, while to economize in weight, its legs would have to be reduced to mere stilts.
At the end he can't help getting a little political - it was 1928, after all:
And just as there is a best size for every animal, so the same is true for every human institution...I find it no easier to picture a completely socialized British Empire or United States than an elephant turning somersaults or a hippopotamus jumping a hedge.