There's only so much breast mass connective tissue can support and still be what we'd call "firm." When you hit the D+ range (with G being practically off the natural scale), gravity pulls the breast downward and the skin stretches, so the resulting contents distribute more or less along a vertical path, resulting in a pendulous appearance.
The skin's stretching tends to broaden the areola and flatten the nipple, producing a pancake nipple area without much protrusion. This happens all the time for women who get large implants.
Because breast tissue is composed largely of adipose-like cells (fatty tissue), naturally skinny women who are not predisposed to weight gain would be poor candidates for exceedingly large fatty breasts.