The WN indicates that they being fat is because genetics, the madam tried diet, exercise and other things but couldn't get rid of that fat. As a result though she developed a sharp sense over any change undergoing on her body.
And much like the pulley that "draws water from a well" disregarding if there's water on said well, the hot spring facility has the effects related to healing, relaxation and beauty. So the madam should be able to shred all that fat after a while.