It's crazy how many of us have this same problem. I agree it's not an easy situation to deal with. You don't want to hurt her feelings or piss her off, but it's something that needs to be dealt with. IMO the best things you can do;
1. Take over the cooking duties in your house, you can't control her lunch or snacks, but you can make sure your family is eating a nutritious meal every night.
2. Get some easy to use cardio equipment for your home. A couple of free weights, maybe a jump rope, if you have the cash and space a treadmill or a stair climber.
3. My wife and I are both competitive with each other, I try and challenge her to things that will increase her cardio. "Race you to the front door" "I can jump rope for 20 min without messing up once" ECT... also get a WII or MOVE or Kinect,, it's amazing how much standing and moving around playing those games, can wear you out, make you sweat and increase your cardio.
4. Lastly SEX increase your sexy, make her get on top and do the work, try every different position you can think of, make her stretch in ways she wouldn't normally, as well increase her cardio. This is a win win! More sex!, plus it burns a ton of calories.
All of those things I can do on my own to try and help out my wife. In the end it's not always enough and we have to have that tip toe "talk."
Oh and lastly the new year is comming up, there is never a better time to politely suggest she make a new years resolution to start working out more, living healthier, and eating a better diet. All of this is just my humble opinion but I hope it helps.