I think I have a reasonable idea of how much you like the curvier girl

. So you have to start from there and assess how important that is to you, whether it's a deal breaker or whether it's just the "icing on the cake" if a girl who fits with you in other ways is also a bit of a stunner in the curves department.
I don't think it's shallow. You are attracted to what you're attracted to. We can't help that. But only you know whether your preference for fatter girls means you're never going to be attracted (or sufficiently attracted) to this girl if she keeps losing weight, or even if she stops right now and stays as she is.
But I would second what other members said above about telling her how you feel. Not necessarily come out of the FA closet or anything dramatic. Just that you liked her figure as it was when you met her and that if she's losing weight because she feels you might like her more that way, then she should rest assured she doesn't need to do that.
Because if I remember rightly, you're quite a fitness fan and she might well think that you admire the same qualities in a girl as you prefer to maintain in yourself. It would after all be the obvious conclusion for her to reach.
Don't get annoyed by her weight loss though - she's not to know that you would rather she didn't until you tell her. But avoid, if you can, telling her in a way which means she starts altering what she wants for herself in order to please you, as that wouldn't be fair - or at least, not this early on in the relationship.
Good luck!
F