With the way they have people sitting on those mounts, turning any faster would result in you going one way and the horse going another.
Turning does indeed seem weird. I just tried turning in small circles on a mount and I agree with what you describe. I would bug report it, if it isn't reported already.
Same could be said for the swing of some of the weapons, some would fall right over. Doesn't change the fact that the mounts turning seems off.
Fighting in games is never realistic. In fact, one could argue such a thing is impossible but also the reality is it's undesirable as realistic fights are not so entertaining and would be over usually within 3-4 moves. Sitting properly on a horse on the other hand, is both achievable and desirable. Suggestion: Pay to have the animator(s) take a week's worth of riding lessons. It would be totally worth it. (Would also help with the horse turning issue I'm sure).
The horse size itself isn't even realistic. I don't look like I'm sitting on a horse, I look like I'm sitting on a donkey. Anyways, doesn't change the fact that the mounts turning seems off.