There ' s a couple of factors that influence breast size and shapeThe genes you inherit and your weight during adolescence edit this prob affects density more than size are pretty strong influencers of final core size at the end of puberty . Then you have your current body fat % which will change things further . Finally there ' s also a development stage when you either first get pregnant or go on certain contraceptive pills . If you never get pregnant or go on that pill you don ' t undergo this . The genetics is pretty interesting , 23 andme did a study which discovered some of the variants that increase breast size The poster contentuploads 201211 G e n e t i c V a r i a n t s A s s o c i a t edwithBreastSizeandCancer . pdf and the article 23501353 . We Humans pick the person to have sex with in a semi nonrandom fashion unless drunk but also the cultural perception of desirability aka whether people in your nation like boobs or bums or ankles varies the ~~ Romans ~~ Greeks for example thought large penises were silly and unattractive so the size of a body part which is not crucial to survival and reproductive success because most of us in the end want to have our kids with the girl who isn ' t crazy rather than the girl with the biggest boobs isn ' t going to be under very strong selection when other factors matter more . Edit Added the article . Edit 2 I ' m told it ' s the Greeks that liked small penises