Fiorella feels thing far too deeply--anger, grief, disappointment--even joy--but knows that expressing these emotions is not socially acceptable. So she swallows everything--everything--and gains weight.
Of course, Fio can only stomach so much, and every now and then, she vomits it all out in an emotional rampage. The emotions, not the food. The food stays around.
OMG! That is the reason she writes too!