I think it is because of the problem of hormone. For example, all the girls in this world have the genes to have the breast development, but this hormone secretion is affected by the environment factors, like temperature, lifestyle. In HK, girls are more stressed and may be somehow, (or poorer air quality, or more artifical food/ chemical) impaired secretion of the hormone. I am now living in Germany and most of the girls here have bigger breast, as they are relative less stress or work here plus good air quality. I hope my guess is true. ^^