How do some very slim girls manage to retain a natural C or D cup?
Genetics. Im slim (54 and slightly over 100 lbs) and naturally big breasted. I had a reduction and am now a 28G. Despite having a low body fat percentage, I still have a ginormous pair of boobs. My surgeon told me that my breasts are almost entirely composed of densely packed glandular tissue. I have very little fat. It sounds great, but there are downsides. Most notably, I have a much higher risk of developing breast cancer than someone whos smaller breasted.