Retagged to medium_breasts because "Any part of the breast would hang over a hand placed immediately beneath the breast" is the lower bound for that. Presumably that means the hand is placed flat against the ribs...

Because if the hand was parallel to the underboob then anything over that threshold would immediately hit the upper bound of "The breast's volume is less than the volume of a sphere with the same diameter as the character's facial area" when viewed from the front.