I agree with ladies here - the woman always knows when she gains weight and it's a touchy topic.
So instead of pointing the obvious to her, just come up with ideas of what both of you could do together - activity I mean like joining the local gym or a dance club.
You can either say you always wanted to learn to dance or that you feel YOU are gaining weight with all this great food you are sharing (or sedetery lifestyle if it's the case) and that you prefer to work it out before it gets worse.
If you don't fancy moving a bit more, then ask her to help you incorporate a new diet, and that you would be really grateful if she went on it with you so you wouldn't feel left alone.
No easy solutions mate, you need to do a bit of work yourself ;)