Sep 11, - What Causes Breasts to Get Bigger? Breast growth is a normal part of development for girls and women. It occurs initially during puberty and may recur throughout life due to pregnancy and weight gain. While changes in breast size are not usually harmful, they can cause physical or image-related issues for some women. Normally not or only slightly. Sex has no direct effect on breast size. Of course boob size is directly linked to gaining age towards 30s, gaining weight or fat.