The bulk of the gluteal muscle mass contributes only partially to shape of the buttocks. The other major contributing factor is that of the panniculus adiposus of the buttocks, which is very well developed in this area, and gives the buttock its characteristic rounded shape. The gluteal muscle bulk and tone can be improved with exercise. However, it is predominantly the disposition of the overlying panniculus adiposus which may cause sagging in this region of the body. Exercise in general (not only of the gluteal muscles but of the body in general) which can contribute to fat loss can lead to reduction of mass in subcutaneal fat storage locations on the body which includes the panniculus, so for leaner and more active individuals, the glutes will more predominantly contribute to the shape than someone less active with a fattier composition.  The degree of body fat stored in various locations such as the panniculus is dictated by genetic and hormonal profiles.