It's not always strictly a boundaries / frame control issue.
There are women with poor impulse control. They might get themselves thin for a while but once they're with a guy the moment they are not completely on their toes they start letting themselves go.
I knew a girl who had a compulsion to "not waste food" and she would just eat all the leftovers even if she wasn't hungry or didn't much like that kind of food to not be wasteful. Fine when she was single and stick thin because she only ever had her own food and could control the portions but as soon as she had a partner if that partner didn't finish all his food she'd start ballooning trying to finish it for him after every meal.
Metabolism issues can play a role. I had a gorgeous cousin who was always in great shape... she had an amazing body... well she married this beanpole guy who loved to cook and eat, and he would be cooking these delicious meals every day, eating tons... he never got fat but she did. He would've had to curtail his cooking to prevent that but the guy loved cooking.
It can also happen if the girl gets depressed and stops keeping on top of what she needs to keep on top of. She might not want to lose the relationship but she just doesn't have the bandwidth to do what she needs to do to keep her guy happy. Happens more if she feels neglected in the relationship or like the relationship isn't heading the direction she wants, but other things can be the cause (career troubles for her, death of a parent, etc.).
What you must do as the man is identify anything you're doing that's contributing, such as:
- Having a bunch of unhealthy food laying around
- Eating out a lot at unhealthy places together with her
- Eating at night around her (you should never eat at night if you want to keep weight off... neither should she)
- Cooking an excess of food around her
- Doing something that triggers a compulsion of hers (like wasting food around a "don't waste food" girl)
- Neglecting her needs in the relationship so she becomes depressed / distressed
... then anything you are contributing, fix that to remove that as a source for her swelling up.
Once that's fixed, then it's all letting her know she needs to cut the weight down, offering to help her, getting her a calorie counting app on her phone, buying a scale for your place so she can measure her weight, checking in with her on it to make sure she's losing it at a steady rate, brainstorming with her on why the weight's not coming off if it isn't. When the issue is brought up repeatedly, and in a constructive way, it gets stamped in most girls' heads that "Okay, this is a pretty important issue, I'd better get to work on this."
If it goes unfixed for too long then you may need to break out the more direct threats: "I want to work with you to fix this but all this extra fat is really just too much for me to handle. I don't like being around overweight people. You used to look so much better. Let's get you back to that."
If that doesn't work, well, then it may be time to cut her loose.
Chase