How to tell girlfriend she's needs to lose weight?

First off I love her and want to be with her forever, and I think she is beautiful,I was in the marines for 10 years and Im in pretty good shape and have a good build. Im not trying to be rude but she is getting pretty sloppy she's like a sack of dirty laundry compared to some of the women who approach me,I wont cheat on her and don't want to be with the other women,what can I say to her and not hurt her feelings, I know how women can take this sort of thing. I love her but it almost hurts our sex life, im not attracted to over weight girls. she has been fit her whole life but has let her self go. any advise would be appreciated . Please don't think im a jerk, I want to marry her but if this weight gain continues I don't think Its fair to me, im in great shape. I don't expect her to have the body of a super model but her weight is getting up there. please help.