There's barely even a hint in the entire game as to what kind of body Momo is rockin' under her scholar outfit. It's fanon, likely due to the genre itself implying that most likely to be case. Nina is already a petite girl on the small side of things, so Momo, an older woman would naturally have to contrast and compensate.

On the other hand, there is some reason to think she has a full figure.

Her father kept a huge stockpile of erotic material stored away. This may allude to his taste in women, and while he may not have limited himself when considering the woman to be his wife, his tastes in reading material likely played a role in it.

The game also mentions that he visited Genmel, the seedy arena town nearby that hosted bunny girls as entertainment. Momo herself is a natural bunny girl (and so was her mother?). We don't know what either of her parents looked like, but Momo's race seems to be fairly rare in this game.

When Momo is selected to be used as a hostage at one point in the game, she mentions that her captors will use her for XXX if the party abandons her. Momo doesn't understand what XXX is, and while the villains at this point in the story don't seem like they'd be especially picky, there is still the implication that Momo could perform that function.

It's so fun to speculate about what amounts to toilet humor, isn't it?