The way you have spoken about your wife and her misgivings doesn't show any empathy for her feelings. Rather than talk about her fears or nervousness in depth, you belittle them by calling it "cold feet." Rather than talk about what, if anything, she finds exciting or enjoyable about being fat and how to manage that against the risks of being so large, you try to make your feelings hers by saying you think she'd enjoy being 500, despite apparently being nervous at 450.
I think like it or not she will love being 500.
Be honest. Do you really believe this? What would you do if she said, outright, that she wants to lose weight--for *any* reason?