Kate Upton’s breasts are a rare breed: plump, perky and awe-inspiringly jiggly. (Just watch her “Cat Daddy” video below.) So why can’t every set of Ds dance like hers? “Density of breast tissue varies,” says Keesling. “Breasts with more fatty tissue are the jiggly ones.” In fact, in a British study of braless female joggers (lucky researchers!), “breast mass” – volume and density, which varies with the ratio of fat, mammary glands and connective tissue –was found to be a more important predictor of movement than size was.