"Past biomechanical studies show that when women run braless, their breasts joggle up and down by seven inches or more and also oscillate side to side...."

"... A 2013 questionnaire provided to women running the London Marathon found that more than a third of them, including runners with small breasts, told the researchers that their breasts often felt sore. But little past research has explored whether breast size and potential soreness affect women’s decisions to participate in exercise or if they influence the kinds of exercise that women might — or might not — pursue. So, for the new study, which was published this month in the Journal of Science and Medicine in Sports, researchers from the Biomechanics Research Laboratory at the University of Wollongong in Australia advertised for women willing to have their breasts measured and discussed. The researchers recruited only women because, obviously, women possess female breasts.... Women with larger breasts, whatever their B.M.I., exercised less on average than those with smaller ones and were more likely to feel that their breast size interfered with moving...."

From "How Breast Size Affects How Women Exercise/As women’s breast sizes grew, their participation in physical activity declined, particularly if that exercise was vigorous" (NYT).

ADDED: From the comments at the NYT, from Mary E:

I had huge girls, and hunted and hunted for supportive sports bras. I was physically active, but high intensity activities like running were out of the question - there was no way I could run down the street without making a spectacle of myself.

Then 11 years ago, I was diagnosed with aggressive breast cancer in one breast. With a strong family history of breast cancer I opted for bilateral mastectomies. I elected DIEP flap reconstruction, which is microsurgery moving fat from the abdomen to the chest to make new “breasts”. I kept telling my plastic surgeon that I wanted nothing bigger than B cups. Bless her heart, she gave me exactly what I wanted. What a relief! I could buy cheapie off the rack sports bras and get full support! I even started running, eventually working my way up to completing a half marathon at the age of 58.

I would give anything to not have had cancer, even to keeping my oversized girls. But since cancer did hit, I was going to do my best to get something good out of it. I wish I’d known what a relief it was to have small breasts, because I would have had breast reduction surgery years ago.