Breast size is mainly determined by your genetics and also your nutrition (i.e. a healthy balanced diet). Breast development usually starts one year before your period starts and is one of the earliest signs of puberty. It usually takes 3-5 years to complete. So if you haven’t started your period yet, your breasts still have a lot of developing to do. Remember also that it can be quite normal to not start your period until you are 16 years old. As hard as it is to not compare yourself to other girls your age, you have to realise that every girl develops in her own time. You will have breasts and periods for most of the rest of your life so enjoy the time you have without them!