The reason why mixed-races are not common though has more to do with society not wanting mixed-raced kids then that mixed-race children are difficult to give birth to. We don't see lots of mixed-race couples with only very few of them have children when the genetics works out. What we do see is the ostracization of mixed-race children and mixed-race couples.

So for the races that do have extreme body differences, I could see them having mixed-race children just fine, but due to society's view of it, there just isn't a lot of opportunity for mixed-race children to be born in the first place.