A study of 16,000 women found genetic mutations associated with breast size were also linked to the disease.
Researchers say some of these are involved in regulating the female sex hormone oestrogen, which can trigger the growth of both breasts and tumours.
While research has linked breast density – the amount of non-fat tissue – to an increased risk of cancer, there has been little evidence of a link with breast size before.
Participants in the study, carried out by a US company, were asked to give their bra size on a 10-point scale from smaller than AAA to larger than DDD.
The genetic code of the women – all of European origin – was read by scientists who looked through millions of tiny mutations in their DNA, called single-nucleotide polymorphisms. Out of seven that were strongly linked to breast size, three were also associated with breast cancer.
Dr Nicholas Eriksson of the California-based genetics firm 23andMe said it was the first substantial link between breast size and cancer, but added much more research was needed before it could be considered concrete.
He said, “Our results identify genetic variants that have an effect on both breast cancer and natural variation in breast size.
“While the precise relationships between breast size, density, obesity and breast cancer remain difficult to untangle, understanding the biology . . . may aid in the development of novel screening tools.”
He said one of the three mutations regulates the activity of the oestrogen receptor gene that plays a vital role in breast growth and in the majority of breast cancer cases.
Another is located in a region of a woman’s genome that often shows abnormalities in those with certain types of breast cancer.
The link was seen regardless of the women’s age, pregnancy and breastfeeding history and genetic ancestry, according to the study published in the journal BMC Medical Genetics.
Little is known about the biology of breast size, which scientists believe is only half hereditary, but high oestrogen levels are known to be a risk factor for breast cancer.
There are many different forms of breast cancer, which scientists believe could be treated as ten different illnesses. Weight, alcohol consumption and a strong family history are all risk factors.
A 2006 study by the Harvard School of Public Health on 90,000 pre-menopausal subjects found larger breast size in slim young women gave them a higher risk of breast cancer in later life.
Those with a body mass index of 25 or less and a bra size of D or larger had a significantly higher risk of breast cancer than those of the same weight with a cup size A or smaller, they found.
However, the study’s lead author, Karin Michels, said the findings did not mean those with small breasts could assume they were safe and urged all women to go for breast cancer screening.
–Daily Mail, London