Hey allohver, although you can post stuff like this here and get different opinions, I would not take anybodies advice. They are not qualified to tell you what to do in your life.
Only you really know this girl. At some stage things were good. What went wrong ? Is unhappiness causing her to eat .... or depression ?
It could be depression mate ... in which case she's gonna need you.
I think you need to have a really good chat and find out what the real problem is.
It aint as clear cut as "plenty more fish in the sea".
If I start putting on weight in a few years I would be devestated if I was left because of it.
It could very well be a catch22 scenario ... she may think you smoke because you've lost interest in her, so eats more. That's why she doesn't like you smoking and that's why she's eating more.
If you really love each other, you WILL work it out.
Maybe cut down on the smoke a bit, go for a few walks with her. Show her that you're willing to meet her half way.
Hope it works out for you