The orogs use a human model. Since the description says they look like a large mountain orc, and a mountain orc is as tall or taller than a human, why is a half-orc model not used?

We use a variety of models for different Orogs. In some cases, we use Half-Orcs, in some we use Humans, and others we use the standard Orc models. There's a fairly limited array, so we try to diversify. Not all creatures are going to be of the same dimensions, regardless of average.