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.

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.