Okay here is a serious question, prompted by Anita`s post above, my partner has let herself get somewhat on the overweight side. In the meantime, I took control of my health and fitness, and have been working my arse off these last seven months to get back into shape.
Not for any other reason than I decided I needed to. The main beginning factor was a health scare and a visit to the hospital.
She does not like the fact I go to the gym so regularly, gets annoyed with me at the least little thing relating to my quest to get fitter and healthier, one tirade culminated in her saying I looked "scrawny, thin and anorexic..." All words used in the same sentence no less.
At the same time, she is going on about how she thinks she looks fine as she is. While I know for a fact she is at least a stone and a half heavier than me and she is 4-5 inches shorter than I am.
She wants reassurance she looks good, but in all honesty I can't tell her that.
So what should I do?
This really, sadly has everything to do with my post.

I'm not sure if I would go as far as SuperMike in assuming that everything about the relationship is going downhill, but she is most definitely insecure and most definitely being combative about your personality change. Behavior like hers is EXACTLY why it can be so difficult to get healthy when you live with your unhealthy family. There's a feeling of "who does he think he is, acting like he's better than us?" Even though your reasoning is completely clear to you - you want to avoid the damn hospital!
Saying that she doesn't look good anymore won't help. She'll just retreat back into her shell and get even more combative. She may have tried diet and exercise in the past and it didn't help, and may be hesitant about getting on board again, because she is afraid of failing. Especially when you're seeing so much success. There's a lot of psychology there, why someone refuses to exercise. I mean, my sister is intimidated by the gym, but she's happy to run outside. I still hate running.
I won't lie, if my entire motivation was to change the way my body looks, I HAVE FAILED. BADLY. I originally wanted to gain weight, and I have not gained a single pound, despite squatting my body weight and deadlifting even more. Although, I have gained a tiny amount of mass in general muscle tone. My clothes still fit exactly as they did before. Changing the way my body looks could not be my motivation then, and as I started to lift more weight, I switched my goal. It was pretty easy, actually. "Look at how much weight I can lift! This is awesome!" "I'm the only woman using the power rack! I'm such a bad ass!" "I will be able to carry my parents out of a hypothetical house fire!"
But the main reason why I started going to the gym and lifting regularly is because I saw my grandmother wither away for a year, bedridden after breaking her hip. (She broke her hip and
never walked again.) And then my mom developed full-blown osteoporosis before she turned 55. Seeing her resignation about it all, that we're fragile and that weak bones and weak muscles are simply our future, that motivates me most of all.
My fiance isn't as fitness-oriented. He's a bony shrimp and I will probably outlift him soon.

But the most important thing is, he thinks it's
awesome that I'm taking charge of my health and my future. He doesn't feel threatened that I'm getting more fit, or potentially meeting hot buff guys at the gym. Our relationship is solid, but more importantly, we're both secure and
know exactly why we're together. It isn't only because of physical attraction.
So physical attraction isn't going to be your carrot. There has to be the solid foundation in both your relationship and her self-confidence. That it doesn't matter if she doesn't lose any weight, but you care about her well-being and that getting healthy together will be great for the relationship, and her. And that getting healthy doesn't mean she's a different person. She'll just be a different, healthier, version of herself.