I think its shallow to coplain about 10-15 pounds, but I am not here to judge you. I, in fact, will help you out keeping this girl hot.
First of all, ask yourself this: If you met a woman you truly love (and only you can answer that), but that woman was 50+ lbs overweight, and rather plain, would you still do her? Keep in mind that you must love her first and like her second when you answer this.
If you do her, you are not shallow. If you do not, then you are.
Ask yourself whether you really liked this girl, or if you just want a weekend booty.
Now, that said, dont tell her she is fat. Everyone has different standards of what fat is, and yours seem to be a bit uptight.
Now, again that said, Everyone also has different needs. Some of us crave an attractive partners because we a) feel complimented by their looks, or b) we like to look at them. So there is a need to have a partner that is attractive/fit (again, all relative). Not everyone has them or even think about that, but is there... I dont mind a chubby girl so long she doesnt cross to the lard-ass territory (for me this is, a curvy girl with "meat on her bones"). Once in lard-ass territory, I have to hide her from friends and relatives.
This is what Id do if I was shallow or had a high need for "hot chicks".
"Honey I like doing IT with you. I liked you from the begining because you were cool, pretty, hip, etc. I was truly attracted to you. I find you attractive when you take care of your body. It makes me really horny to know that you like yourself enough to sweat the extras."
This only works while they are gaining weight. Once they are fat, if you say this, you are controlling, selfish, and you better bring her some ho-hos... Otherwise she will keep nagging. Any mention of Atkins, gastric by-pass, or lean cuisines and they will sit on you.