@BHG here's the thing. It sounds like the relationship is over and she's just an animal you keep around out of habit and legal problem reasons.
If there's no hope, you don't want to give her anything that implies self-improvement. Like smaller sized panties. Or fitness stuff. Or anything calling her fat.
I'm just thinking here's a dildo you filthy animal. Keep busy or something, pet.
@BHG Here's another idea. Instead of anything that insults her as fat, do the inverse. But you also want to be spiteful, so it can't be something nice.
Get her a big bag of chips. Just shitty junk food. Maybe something from Taco Bell.