While the primary function of female breasts is to nourish a baby, they have been a symbol of beauty, eroticism and femininity since time immemorial. You can't blame Madison Avenue and Hollywood for that. What they can be blamed for is the emphasis placed on size and perfection. Size and shape is variable, probably the most variable of any body part. Not only does it vary from woman to woman, but it varies both throughout a woman's lifetime and throughout her monthly menstrual cycle.
Breast tissue is hormonally sensitive, especially to estrogen.
Size does not influence the amount of milk you can produce and size definitely does not influence your sexual ability for arousal or response. You cannot enlarge the size of your breasts with exercise because there is no muscle tissue within the breast. So-called 'bust enhancement creams' will not enlarge your breasts either so don't waste your money. The ONLY way to change the size and shape of the breast is by surgery. Each breast is also two different sizes in each woman.