Have you tried Chicken Soup For The Cat Lover's Soul? They have a light formula, it really works! It smells so yummy! I haven't met a cat yet that refused to eat it. The kibble is fairly small and softer than most cat foods.
Some allergens can be found in food, most commonly corn, wheat, soy, beef, and dairy products. Chicken Soup contains no corn, no beef, no soy, no wheat, and very little dairy.
My 3 year old male cat, Peppurr, was 20 pounds last November, he is now 18 pounds. He is slowly burning off that fat, the food gives him so much energy!