@OP, as has been said, no one likes everything about their SO, but if weight is the worst thing in your relationship (assuming you don't have an unhealthy obsession with weight) you should count yourself lucky.
How to address it? Well, I would say don't listen to the obviously jaded married men here. And the one woman who suggested you be completely supportive and somehow that will magically communicate your concerns, yeah, I wouldn't listen to her either. Listen to yourself....chances are you know what you have to/want to do and came here for validation/confirmation of your concern and what you feel you should do. You know your wife best, what type of motivation she responds to and doesnt't, her ability to commit to change, he desire to change, and so on. All of these are significant factors in the success of a long term change, but none of which we as board members know anything about. Look up some habit changing techniques such as doing a purge, i.e. get rid of all junk food in the pantry, fridge, freezer, and stock up with healthy stuff. Start a routine of introducing activity slowly into your relationship (morning walks if schedules permit - or make them permit it). Be honest. Tell her you want to want her like you used to, but you don't with the extra weight gain and for the survival of the marriage you want to work on it together. Ask if there's anything she wants you to work on and agree to make the changes together. If she can't accept how you feel and doesn't want to change for you, her, and the marriage, then address that problem first, not the weight, and likewise if you react negatively to what she says about you. My 2 cents.