First I would think an indirect approach would be better, such as asking if she'll go to the gym with you cause you hate going alone or something like that.
If that doesn't work, then telling her she's fat and ENCOURAGING her to lose weight is better then the alternative of breaking up with her and giving her a fake reason why.
Now I'm assuming we are talking about an overweight girl here, if she's like, 130 and you would rather have her at 110 maybe you should reconsider your priorities.