Love her the way she is and respect her decision.
If she is unhappy being big, then you have to understand that it wasn't for her. Sit down and talk with her about it and she will put you in a better picture.
Do not try and trick her with weight drinks or anything else, she will end up resenting you and walking out. And "but i love you the way you are never works" because once a woman has a bee in her bonnet you are not going to change her mind unless shes indecisive of course.
Anyway for now just stop all weight conversations and let her diet, tell how good shes looking and give her lots of love, affection and encouragement. Then after 3 weeks when the pizza cravings kick in, if you are lucky the diet may end. If it doesnt just accept you still have a beautiful wife, just slimmer!!
17 years