Conservation of mass dictates that if a slimegirl so wishes to increase her breast and/or hip volume, her height must undergo an inversely proportional change in magnitude .-.
Unless she has other ways of increasing her mass, such as through absorbing water. >:V
There’s an instance when Suu expands bazongas without a perceivable fluid intake (i.e. no drinking, eating nor assimilating mass through her skin) and even manages to make herself taller.
So unless you’re trying to tell me slimegirls can operate on a molecular level and absorb humidity out of thin air