A breast size that is slightly above average. In America, the medium to larger C Cup sizes are the optimal range of breasts, as they strike a happy medium between too small and too large to adequately grope. By Japanese measurements, however, a C is the equivalent of an American small B, so As are something truly pathetic in Japan.
1: Damn! That bitty got perfect titties!
2: I'd say she fills a C Cup pretty nicely.