Micromastia also called hypomastia , breast aplasia , breast hypoplasia , or mammary hypoplasia is a medical term describing the postpubertal underdevelopment of a woman's breast tissue. Breast development is commonly asymmetric and one or both breasts may be small. This condition may be a congenital defect associated with underlying abnormalities of the pectoral muscle as in Poland's syndrome  , related to trauma typically surgery or radiotherapy or it may be a more subjective aesthetic description. Self perceived micromastia involves a discrepancy between a person's body image, and her internalized images of appropriate or desirable breast size and shape.
Most body parts, like your arms, legs, feet, and ears, grow to a certain size and then stop. Your boobs , on the other hand, are a completely different story. Your breast size and shape can go change throughout your life. Of course, your boobs tend to have a standard size that you consider your "normal. Here are the biggest factors that influence the overall size of your breasts. Your genes dictate your hair and skin color, how tall you are, and a bunch of other things including, yup, your breast size.
You and your boobs go way back. You probably know them so well by now that you could pick them out of a lineup if you had to. That's because boobs are a little like snowflakes—each with their own unique shape, texture, and characteristics.
My 12 year-old sister has breasts that are bigger than mine. Is something wrong with me? What causes small breast size in women? Please help!