OK, if this is going to drag on I'm going to get serious about this. I memorably discussed how sexual relations between Au Ra are possible despite the difference in their stature, so no worries.
From a physiological standpoint, Au Ra females have developed breasts likely for the main reason human females are speculated to have developed breasts: to garner male attention even when they aren't currently capable of producing offspring. Permanently developed breasts are a unique trait among humans; no other mammal has this trait. The only other known hypothesis for why this is the case is that it helps nursing infants attach to the nipple. (Despite what some people may believe, larger breasts do not indicate a female is capable of producing or storing more milk. That has nothing to do with the amount of mammary glands females have, which is largely the same regardless of breast size; the visibly developed breast of a human female is, largely, fat and connective tissue.) Of course, whether evolution is a thing on Hydaelyn remains to be seen, so all of this scientific conjecture could be invalid. I think I remember from somewhere that Hydaelyn (the current planet) is about 10,000 years old - that's not enough time for evolution to produce major changes in organisms.
Nothing about Auri origins themselves have been divulged, beyond their creation myth alludes to a Dawn Father and Dusk Mother that the Raen and Xaela believe themselves to be respectively descended from. Furthermore, despite the fact that they have scales andhornscranial projections, Au Ra are not related to dragons, demons, or reptiles in any way (that we know of). They are implicitly mammals like the rest of the playable races, so they likely reproduce and take care of (nurse) their young in the same manner.
From a design standpoint, it's primarily to help people identify with the characters. People empathize easily with humanoid characters (hence why robots with humanoid shapes are easily sympathized with), and visibly developed breasts are part of the human female form.
P.S. Sorry for trying to be a "moment killer," so to speak, but it's kind of an inane question. Plus I like providing infodumps.