It’s because of something called the the square-cube law. There’s a famous quote:
> “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. A rat is killed, a man is broken, a horse splashes.” — J.B.S. Haldane
https://www.physicsforums.com/threads/a-man-is-broken-a-horse-splashes.585757/
https://en.wikipedia.org/wiki/Square%E2%80%93cube_law
tl;dr: as things get bigger, surface area and volume increase at different rates. A creature that is 30% bigger than another may way three times as much. This has a ton of physical implications.
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