Fat seems the most plausible answer.
"Fat is laid down on breasts as much as thighs or bottoms. We are experiencing an obesity epidemic, so the increase in women's measurements isn't that surprising," the Daily Mail quoted Professor Michael Baum, an expert in breast cancer, as saying.
But that can't be true for those who are slim elsewhere.
Dr Marilyn Glenville, a nutritionist specialising in women's health and hormones, said, "It's clear that we're not just talking about fat, but increased levels of breast tissue, too."
"So we have to look at what stimulates breast tissue growth -- and that's oestrogen, the female sex hormone. Oestrogen is what changes our body shape during puberty."
Glenville said that 'breast-enhancing' supplements could be to blame. In addition, today's young women were born to the first generation of women on the contraceptive Pill.
She added that pregnancy and breastfeeding have a positive effect because they control the hormones, which stimulate the growth of new cells in breasts but with more women opting for late pregnancies, there is an increased number of monthly cycles and therefore, more exposure to oestrogen.
And there's no way to avoid them because they are found in items we use on a daily basis, just as cow's milk.
"Pesticides, plastics and cosmetics are my main concerns. The same goes for xenoestrogens in the deodorants, make-up and moisturisers we use," Glenville said.
"We apply them to our skin and often directly on to the breast. Our skin absorbs those chemicals readily. It is not inconceivable that those chemicals stimulate growth in breast tissue."
We also live more sedentary lifestyles these days, which may mean we metabolise these hormones less quickly, she added.