1. They stand on 4 limbs, rather than 2.

2. That would be assuming that human legs are sized at the max capacity they can carry, which is not true.

Regarding 1, horses in particular are known to jump off two hind legs and land on their two forelegs, putting a lot more force on them than their stationary weight.

On 2, I believe that's the GP's point too — human leg bones (or horses') can obviously take a lot more than their standing/walking/running weight.

	▲	ndr 2 hours ago \| parent [-]
		Agree re 1, but point 2 still stands. They can take more stress than standing, but the scaling law is still there. If you scale linearly height/section the max stress won't scale linearly because the weight will scale cubically.