There are several potential causes for why your breasts may have become smaller. Breast size is largely determined by genetics, hormones, and age. During puberty, your hormones fluctuate, which can cause your breasts to change size. Additionally, as you age, your hormone levels will change, which can also affect the size of your breasts. Other factors such as weight gain or loss, diet, and lifestyle habits can also play a role in your breast size. If you’re concerned about why your breasts have become smaller, it’s best to consult a healthcare professional.