Why don't I have big breasts at age 15, and why don't I have a bigger butt?

There are several important points to be made in answering your question. The first is that you are fifteen years old, and while women mature faster and attain full growth earlier than men, in general, it may well be the case that you have not yet finished growing.  Secondly, for many women, the size of their breasts, and also their buttocks, is often correlated with their overall body weight and percentage of body fat.  Adolescents tend to be leaner than adults, but with advancing age, and a corresponding increase in weight, it may be possible that both your breasts and buttocks may increase in size.  Also, many women experience an increase in breast size with pregnancy and breast feeding, and this change in size may continue long-term, even after the cessation of breastfeeding. The point is that the size of your breasts and buttocks are not static over your lifetime, and may very well change over time from what they are now at age 15. Younger patients like yourself can also perform exercises in the gym like squats, deadlifts and barre routines that can have a positive effect in terms of shaping and toning the gluteal muscles that lie beneath the skin and fat of the buttocks. Lastly, plastic surgery can alter the size, shape and symmetry of your breasts by breast augmentation, and your buttocks by gluteal augmentation (with buttock implants) and/or fat transfer ("Brazilian butt lift"). However, at age 15, you cannot legally make the decision to have plastic surgery by yourself. You must have the consent of your parent or guardian to proceed.  As a plastic surgeon, I would be hesitant to perform such procedures on you right now, since they are of a purely elective nature, and, as previously mentioned, you may not yet have finished growing, nor may you yet have the emotional and psychological maturity to comfortably handle the surgery and its sequelae.  While I believe it is good for you to be aware and have an understanding of the surgical options of which you may avail yourself in the future, I think the best counsel for you is to relax, be patient, and focus on the variables you can control right now, like eating a healthy diet, getting plenty of exercise and enjoying your adolescent years.

Genetics and age is the answer. Features are determined by what your parents gave you in your DNA so that is a good guide of what you can expect eventually but you also have a few more years of development left as well so things may still increase in size yet.

Not all of us get big breasts and big butts. That is why God made plastic surgeons. You are probably growing according to your genetic makeup. If the women in your family do not have big breasts and butts, you are unlikely to have them also.