Imagine an animal made twice as long, twice as wide and twice as tall — the same shape, simply enlarged. Its weight depends on how much body there is, and body is a volume, so the weight becomes eight times greater.
Now look at what has to carry that weight. A bone holds a load across its cross-section, and a cross-section is an area, so it becomes only four times greater.
Eight times the weight on four times the support. Every part of the bone now carries twice what it did before. Double the animal again and the load doubles again. Long before an animal reaches the size of a house, its legs must become disproportionately thick, or they break.
That is exactly what large animals look like. An elephant's legs are thick columns held almost straight beneath it; a mouse of the same shape would look absurd. The mouse can afford thin bent legs because it weighs almost nothing.
Nothing in this argument is about bone. Swap in any material you like and the numbers do not change: eight against four, whatever the animal is made of.
The same rule limits insects, decides how tall a tree can grow, and explains why a small creature can fall a long way and walk off. It is not biology. It is the fact that volume grows as the cube of length while area grows only as the square.