For the record:
I'm very thin. And i Clean my plate ALL the time
Food on plate = Eat
If she wants to lose weight . . . she will. There is really nothing you can do or say . . . just don't make it an issue and she'll do her own thing.
As to your fear of letting herself go . . . she is obviously already adressing being fit, so i would try not to worry about that too much.
so in short. Just date her, have fun, get to know and love her . . . She'll lose the weight or she won't . . . really no impact that you can have there.
peace,
Sweetpea