FemaleFatAdmirer:
Don't convince her. Just stay with her and treat her right. Women get fat when they feel comfortable in a relationship, and they get fatter as they age. So don't worry, you'll end up with a fatty.
This. Literally, make her happy and secure and make sure she knows it's for the long term then you'll more than likely end up with more to cuddle.
But remember, once she starts to gain, make sure you she can see your increased attraction. And of course, when asked be honest.
I was amazed at how happy my wife was when given a carte blanche to be any size she found herself according to her natural habits. Again note that it's a proper carte blanche which means if she finds herself slimming down to 100lbs again (she's currently 170lbs) she knows she's safe in our relationship.