Gaining

So basically you introduced her to a brand new idea/concept and then gave her an ultimatum. I wouldn't do it that way, but I hope it works out for you and her, in the end. :-)

In print it sounds like a selfish ultimatum and I hope She knows you well enough that your "request" is well intended. It's really about what she needs not what you want.
Hope it works out.

You know, there might be a different approach needed here. You're applying your logical solution to her emotional "problem".

We guys tend to go into fix-it mode. It's our nature but not always what is actually needed. I suspect it's more than her size or weight alone. She may lose weight and still not like her appearance. She may be in conflict... Perhaps she herself is okay with her size but feels obligated to hate it because society says so.

If you really truly desire her to be happy, and I believe you do, she may need you to understand her. Understand her unhappiness from her emotional perspective, not your logical one. You need to help her see past the surface issues to the deeper ones. If she's that unhappy, why has she not dieted herself? It's not like she's never heard of them.

Open honest safe communication is what I think you two need, where she can talk without being judged or subjected to solutions shes not ready to hear. Of course, sometimes, we do just need a stern kick in the pants to get us going, too. I hope that makes sense and I hope it all works out good for both of you.

Your behaviour is very selfish, sorry to be that direct.
You are placing your fetish above health and desire of your partner.
This could work for a few years (keep in mind that you are dominant in this decision and she was not strong enough to resist and place her will) - but later she could regret and the base of your love/friendship/relationship vanishes.
Getting old happily looks different from my point of view - an ultimatum should never be used between lovers for such a thing.

I don't understand why she has to do one thing or another. If she diets, she will regain the weight. If she becomes a feedee, she will gain weight. Maybe she is happy (as she can be) in the middle staying at a steady weight.