Of course there's a limit, if she'd doubled in weight since you started dating her, it's probably worth carefully bringing it up, however, this seems to come from the 'men are perfect and women are there to serve and amuse us' line of non-thought.
To me, the worst answer to 'would you still love me if I lost my legs in an accident?' wouldn't be a simple 'no' but 'of course, but if you put ON weight, I'm fucking outta here!'
I think it all comes down to whether love is involved, if it is, you fall in love with a person, not a shape, whereas if it's purely sexual, and you can only get it up to a size zero, then go ahead and trade her in for a new model. With any luck your own metabolism will shift and the love handles will show and she'll be off too.
To me an overweight woman is more attractive than one who fusses about her food, for fuck's sake eat something.
As Alan Davies of QI fame once said 'I offered my girlfriend a cashew and she said no, its one cashew! She said they're fattening, and I told her ' you could have one, pick your nose and you'd lose weight!'