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.
2. That would be assuming that human legs are sized at the max capacity they can carry, which is not true.
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.
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.
But, their bones could also have larger cross-section and a giant could be stockier yet still appear "human like".