I think basically it's about a country rapidly developing, percentages of population being more sedentary through either not needing to work or doing computer/office jobs and also having more money for richer foods. richer being a relative term re: food, but we're talking about fat content (different types of fat), total calorie intake, junkfood, etc.
West african natural propensity for proportionally large breasts is exploding because of these factors. In East Asia, japan was first able to embrace more sedentary lifestyles and western style foods and they are now known as being generally quite capable of huge breast growth. Korea is getting MUCH chubbier but also much bustier, though that is spread very unevenly. The genes of Koreans are NOT as homogenous as people would like to believe.
It is just a thought but it seems like low quality food (high sugar/processed general calories/cheap oils) does not generate the fullest breast growth as the obese people I see who are in obviously poor health(poor skin, poor gait, bulgy stomach) tend to really not have proportionally big boobs. Women who are fit/apparently not lacking in healthy nutrients, yet fat more often have big boobs compared to body size.