There is no hard biological reasons why male ducks have such a nicely colored feathers, except because females find them attractive. (There might a weak link between this attractiveness and his physical fitness, but, well, it's a weak link. And some animals actually can get in problems for focusing on its "sex appeal" this way - Elks or Reindeers, IIRC, sometimes grow antles that are too big to be practical or healthy)
The breasts of most female monkeys aren't particular big. A lot more noticeable are usually the buttocks. A suggested reason for this difference between humans and monkeys is that when humans learned to walk upright, the behind wasn't as much in the eyes as the breasts, and so the attention shifted there.
We might be lucky that the attention didn't get shifted just to the head, otherwise we might now look like the coneheads . (I suppose it was easier to grow fat then bones.) Or maybe this would actually have been better for humans... more space for the brain? (But then, the brain is already consuming a lot of energy, we might get new problems...)
Still, some attention to the head remained - a beautiful face and hair are also very important (unless I am unique in that regard.)
Dragonborns also walk upright. I suppose female Dragonborn don't have the typical fat tissue and more muscles up there then humans. They might also grow more beautiful scales on the head, which to humans might also look vaguely female...