The study found heavier women had about a 30 percent increased recurrence rate and a 50 percent increased risk of death.
The reason? Scientists say it could be because certain hormones that are linked to body weight may also fuel tumor growth in the most common form of the disease. That's known as estrogen receptor positive breast cancer.
Previous studies have linked obesity to a higher chance of getting breast cancer and worse outcomes in women who have already been diagnosed. But scientists say these latest findings make the post-diagnosis clearer.