The variation in breast size is probably due to the diet of the chicken. I prefer the thigh area.
Why Asians/East Asians and not Orientals?
There is a huge difference in the appearance of Orientals to Asians.
Well, sure. Some are rugs, some are people.
Persian rugs?
Winters are pretty cold in northern China, Mongolia, and northern Japan where A cups and small waists are the norm. Plenty of milk producing livestock around, too.
Another human characteristic with great variability (among all races, despite Bell Curve’s misguided claims) is intelligence.
We’ll skip the speculation about intelligence - breast size correlation. 
Granting the rarity, yes. In college, I regularly interacted with a young lady who was otherwise so slender her ribs showed above her (ample) cleavage. I am pretty certain there was no artificial enhancement involved, as this was the era of Women’s Lib - meaning exactly what you think it does. :rolleyes:
One hypothesis I have not seen mentioned is the possibility of humans having been influenced by a span of littoral, semi-aquatic living. (Supported by some characteristics humans share with known aquatic and semiaquatic animals, but other great apes do not.)
In this theory, the enlargement served a functional purpose of allowing infants to “get a grip” in the face of body hair reduction in order to feed more easily.
Possibly because the ram came from a poor milking mom? In dairy industry, the males are kept based on how good milkers their dams, and later daughters and grandaughters, are. It is considered poor managing to breed your top milkers with sperm or rams whose ewes arent top milkers.
Actually, I think that it is well-established that larger breasts on women tends to decrease the intelligence of men in the vicinity. 
Thus proving that having zero breast size leads to the greatest variability in intelligence.