The researchers asked 400 women aged 50-plus to exercise regularly.
Half exercised for 30 minutes a day five times a week - meeting the guideline set by various health bodies around the world. The others put in twice as much time in the gym, clocking up 300 minutes, or five hours, of activity a week.
These women shed more fat over the course of a year, including abdominal fat.
Not all fat is equal and pot bellies are thought to be particularly unhealthy because the fat wraps around the body's organs and produces hormones and other substances that raise the risk of health problems.
The researchers from Alberta Health Services, the Canadian province's health authority, estimated that the women who followed the exercise guidelines cut their odds of breast cancer by 4.6 per cent.
Doubling the amount of exercise reduced the risk of the disease by 6.9 per cent.
Writing in the journal Jama Oncology, the scientists said while meeting exercise guidelines helps, exceeding them is better.
Lead researcher Dr Jessica Friedenreich said: "Post-menopausal women may derive unique benefit from exercise because there is a tendency for weight gain and abdominal weight gain after the menopause.
"Moreover, body fat, abdominal fat and adult weight gain increase the risk of post-menopausal breast cancer. Our findings provide a basis for encouraging post-menopausal women to exercise at least 300 minutes a week."
British charity Breast Cancer Now also advises women do more than the recommended two and a half hours a week. However, rather than the five hours being promoted by the Canadians, it favours three and a half, or half an hour every day.
The charity's Eluned Hughes said: "Being overweight or obese after the menopause increases breast cancer risk, and this study confirms a lot of what we already know - that physical activity can help to reduce obesity.
"Four out of five breast cancer cases occur in women over the age of 50, so it's important that we work to reduce the large number of people developing breast cancer after the menopause."
She added that women needn't spend hours in the gym - any exercise that makes the heart beat faster and raises body temperature and breathing rate counts.
Tom Stansfeld, of Cancer Research UK, said: "While this study doesn't actually look at breast cancer risk, we know being active could lower the risk of post-menopausal women developing the disease and it's also a great way to lose or keep to a healthy weight.
"Whether it's walking the children to school, cycling to the shops or work, building activity into your daily life can help you be more active and stay healthy."
Dr Friedenreich said everyone could benefit from doing a bit more.
"We know that increased weight and obesity are associated with a number of other cancers which include ones that affect men and younger women as well," she said.
"From the public health perspective, trying to get people to do 150 minutes a week may seem like quite a challenge. But if people are already physically active and are able to do more, it is even better to do 300 minutes a week."
- Daily Mail
