And developed cat acne on her face.

The diet forms have MORE carbs, and this sets up the weight gain.

The whole secret we found for all 3 of our cats (I found by accident) is that
1) you need good quality omega-3 fatty acids in the food --cats need this

2) watch out for corn. It is first listed ingredient in many foods including Science Diet --although Science Diet did reformulate to imitate IAMS

3) high protein and fat are required by cats more so than dogs

Our previously 18lb cat is now 15lb, and holding there. And she is 19 now. I switched her to IAMs 10 yrs ago when she developed acne on Science Diet Light. She lost weight on the shrimp. Our middle cat (13yrs normal wt) wants turkey/chicken scraps for her treat, and Oreo the youngest eats rabbits that she catches!-- I know--gross. She is the only one of the 3 who "eats out".
Her treat is very strange --whipped cream. She is the thinnest of all 3.

Our vet cautioned us.... do not allow a FAST weight loss for cats. Fast weight loss can kick them into a fatal liver toxicity. She was very worried at first when Sheba lost her first weight, and monitored her liver functions.
A cat who suddenly stops eating for any reason, may die from this liver thing, and not from the illness that created the anorexia. (like a severe hair ball attack).