Sensual touching when she's eating, anything positive even if it's just a pat on the head or cuddling will make eating a turn on and start to make her unable to control cravings.
Further reinforcement such as rewards for stuffing herself to the limit and becoming cold and distant when she doesn't will make her overeat at every opportunity.
Outright sex while eating or tying her down and using vibrators on her while force feeding will turn weight gain into a sexual fantasy you can use against her as a way to tease her and manipulate further.
If you're not quite on the same page with her as far as making her fatter yet, then there are subtle things you can do.
Bigger plates trick people into eating more than they normally would because it's typical to go by how full your plate is for your servings rather than how much you actually put in it.
If you're looking to get her stuffed make the meal have a time limit, the "fullness" feeling is delayed by about 10-15 min from when you actually reach it so making her eat quicker will cause her to go past her limit before she starts to feel she's had enough.
If you're looking to make her snack harder just keep her distracted all the time when eating. It sound obvious but it's worth pointing out that you eat significantly larger portions when snacking if your attention is totally focused on something else.