Have an open and honest discussion. Tell her about how you feel. Let her know you're on board if she wants to lose weight. You love her now, you'll love her then. You love her body now and you'll love it then. And if it makes her happy to be there, you will be happy for her. Reach an understanding. Listen to what she has to say. It's just like working through any other touchy subject in a relationship. Let her know you value the relationship and HER above all else. And you're here for her, to listen to what she has to say.
Let us know how it goes, friend. Best of luck.

Closet feedee - me too. I'm coming out now so to speak. (Online at least.) Never purposefully gained. Stuffed every so often, but now I'm here to see where the journey takes me.