When people change dramatically, their standards follow suit.
What that means is, if you get her while she's fat, and you get her to drop the weight, unless you also drop a lot of weight / pack on a lot of muscle, expect her to thank you for helping her lose weight by finding herself a more attractive man (that is, her dating options will go up as soon as she's more attractive, and the men she couldn't get before will now be interested in her).
Also... the general trend for women in relationships is to gain
more weight, rather than lose it
When selecting someone for a relationship, only date a girl you can accept everything about. Nobody changes anything unless she decides to change it yourself, so if you go in thinking you're going to get her to change one thing and then it'll all be fine, you're going in with a mentality that's setting you up for barrels of disappointment. Assume that nothing will change, and decide if you're happy with her like that, or if you aren't.
Chase