With my ex, it was simple -- I told him how I really felt about it, "I'm concerned about your health." It was a fair statement, he was 400lbs and diabetic. He also knew he needed to lose weight, and appreciated my involvement in supporting him getting healthier. I was willing to work with him to completely overhaul the food we had in the house, our activity level, etc.
With my current husband, he doesn't need to lose weight -- he has a little bit of a belly, but very little, and he's 52 so he's entitled. However, I want to keep him around for a long time -- we have a 9-month-old baby and just found out there's another on the way -- so I've told him, "I would like to exercise more. Would you be willing to walk with me in the evenings?" It seems to work very well.
That said, if the weight is not a health issue, and more of you're not attracted to that little bit of spare tire, think about it for a moment. How would you take it if he approached you and said that he felt you need to lose weight? Before you say anything at all, just think about how it would feel if it were from him to you rather than the other way around.