I am very aware that this is a fantasy game. And know that appearances etc. mean nothing ingame.
If you look at how the orcs are built etc and how intelligent they are (read the books), how is it possible that humans can win? (note: I said cán win)
Sure one orc isn't the same as another. Same goes for humans. But really look at those muscles. How big they are. How combat trained they are from the moment they are very young.
Look at the humans: Sure we might have had (in the past atleast) better fortifications. But humans are generally a little slender and less muscled vs the orcs. They don't start training with weapons as early and under less hard circumstances. They might get better schooling, but this is hardly noticable in the books. Sure there are dumb orcs, but there are dumb humans too.
So if you look at it like this. How is it the humans have survived thusfar?