Champion
The more you encourage her to lose weight, the more you'll reinforce in her mind she's ugly to you. That's a horrible feeling for a woman to have. Why should she have to "sacrifice" anything, just because you are attracted to thin women?
If you can't stand her appearance any longer, it may be time for you to move along and find yourself a blow-up doll. If appearance is so important to you, you are the one who may have to sacrifice substance for fluff. Beautiful women who attract a lot of men (like yourself) can be very high maintenance and think only of themselves and what you can do for them. They can also be boring when it comes to conversation.
If your woman now is a good woman inside, you don't deserve to be with her. Set her free so that she can find a good man who has the strength to find her inside beauty and take care of her needs.
Not trying to be harsh, but looks fade - it will happen to you too.