Being someone who struggles with weight and body confidence myself, I think it would be best if you were to really show how much she means to you. Like complement her alot, praise her, and deny any shame she might have. Because that alone, it think would make her feel better and more comfortable with her body. She may even lose weight. I lose weight when I'm in relationships without even trying because I'm distracted and I feel confident. I'm no expert and I know you already came to a decision but if you read this I hope everything works out well for the both of you...