Just add water!

Tiny shows off his slimming self

Recent studies have shown that adding water to cats' food slows weight gain and increases activity.
Cats that consumed a dry diet with approximately 50 percent water moisture gained weight at a slower pace and were more physically active than cats on a dry diet with 10 percent moisture.
Maintaining the new slimmer profile after a successful weight loss diet is difficult for many cats and their owners. This study shows that even cats on dry food-only diets regain less weight if water is added.
You can build on your cat's increased acitivity by encouraging your cat to exercise. 
I don't recommend that any cat is on a dry food diet only. At least 50% of the diet should be wet food as this prevents dehydration and long term kidney problems as well as helping maintain a slim, sleek body shape. Wet foods include raw meat or bones, and canned and sachet foods.
If your cat won't eat wet food then adding water to dry food and not leaving the bowl down all day will help fight those extra pussy pounds.
Raw meaty chunks, meaty bones or a few dry special dental biscuits keep cats' teeth and gums clean and healthy.