Champion, at that weight your girlfriend is at a perfect weight for her height (if your guess at her height and weight is true).

And what does she think about her weight? Does she too think she is overweight? If she is overweight, does she herself mind?

A few months ago I too was slightly overweight (I have eating issues, kind of) and I hated it and I especially hated it when some one I know took it on themselves to point it out... it was none of their business. So the weight of your girl friend is none of your business either. If she wants to be overweight, let her be. Only if she's finding it distressing should you help her. Trying to make her go to the gym and do this and that just to lose weight would put me off too, if the same thing was happening to me. Your judgement about her weight may not even be accurate. Women have different body shapes. Like some other people on this thread have said, perhaps you could see if there is some other friction between you and your girlfriend which is causing this tension over the weight issue.