Most studies have found no association between breast size and breast cancer risk. All breast cancers develop in the cells that line the ducts or lobules – the parts that make milk and carry it to the nipple. Regardless of their breast size, all women have the same number of these. What makes breasts bigger or smaller is generally the amount of fat and stroma (fibrous tissue), which research shows have little impact on cancer odds.
However, tumours are harder to detect in larger breasts. Large breasted women often have lumpier breasts and detecting suspicious lumps until they are well advanced may be difficult. Compared to that, it is easier to detect cancerous lumps in small breasted women.
One carefully conducted study did find an increase in the risk of breast cancer among lean women with larger breasts. In this study, two thousand women were grouped according to their bra size before childbirth. Women who were lean (chest size less than 34 inches) and had larger breasts (size B or C cups) were at significantly higher risk of postmenopausal breast cancer relative to women of the same chest size with an A or smaller cup size. Women with other chest sizes had no association between breast size and breast cancer risk. However, we need more studies to validate this theory.