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
Couldn’t slime girls potentially alter their density to make them look bigger or smaller yet keep the same mass?
the defining property of fluids that distinguishes them from gases is that fluids are nearly incompressible, i.e. their density changes maybe by 1/1000 over a wide range of temperatures and pressures
conservation of mass would not prohibit such behavior, but such slime girls would rather be steam girls and could only exist at 200 times the atmospheric pressure and temperatures of 400 °C (752 °F)