One of my moms cats is a 25 pound tabby. He's a big sack of potatoes. He eats Science Diet Light, 1/2 cup per day in 2 increments of 1/4 cup 2x per day. Her other cat, white female eats the same amount of food, same kind of food, and she's 10 pounds (she's on the large size but not at all overweight, she's just a bigger boned girl). We have tried perscription food for Tubby (one of his fitting nicknames) and he just will not lose weight. It's quite genetic in him. He very rarely gets table scraps. My mom and I both worry about him, but we will just have to accept whatever health issues arise from his obesity.