Is she a balloon molly or regular molly? Balloon mollies really only show when they get close... The gravid spot will become darker and wider, and she will become boxy. I assume it is the same as regular mollies.
There is no such thing as a "fat" molly. Either she would be bloated, or pregnant. Considering it is a molly, 99% says pregnant :)
Breed for the breed, not for the money; the words any REAL breeder would understand.
You must know nothing in order to know all, for all is nothing and nothing is for all.