The standard against which optimal breast size is gauged in seeking a suitable mate, etc.
The optimal breast size is equal to the amount that can be fit in one's hand; any less is not enough and any more is excessive, wasteful, and unnecessary.
The curve follows a sharp Gaussian/Cauchy–Lorentz function and distribution.
f (x; 0,1) = 1 / π (1 + π^2)
This is informally known as "The Hand Rule."
Person 1: Wow, she has exquisite breasts.
Person 2: Eh, they're too large.
Person 1: Nonsense!
Person 2: Where do they fit on the Breast-Volarity Curve?
Person 1: Ahhh, good call.