Should I be honest with my gf about her weight?
Probably depends on her weight. What do you plan on telling her?
I want to tell her that she's too fat
I understand that you want to be honest with your partner, which is a good thing.  However, I'd suggest saying it in a way that sin't so mean.  You shouldn't be mean to your partner about their weight or they might get depressed!
She's a smart women so she'll understand what I'm saying and change her ways. I'm doing for her own sake.
I think she will be sensitive to this topic, but I understand your concerns because obesity comes with a lot of health issues. Maybe instead of being blunt about it, just ask her to go to the gym with you so that both of you can work on your weight goals together. That is why you wouldn't have to straight up tell her she is fat.