Just to give my 2 cents worth... there ARE many asian girls who spot Ds and above.It's only that most of them have faces like pugs or harpies. Well, the majority of the remainding who Do have pretty faces are either too short or have really bad bust to waist ratio (in short, BBW). What we have in the end are a small handful of creme de la creme. When some of them are seen by professional photographers or head hunters, you get people like Harumi, Yoko, Rio, etc.
Personnaly, i don't think those mentioned models had BA. Augmentation is still a taboo to asians. If it wasn't, big-breasted asian porn won't be such a rarity, would it not?