Welcome to the forum. My son has a fat cat too-one obese, one fat and one normal. He changed the cats' food to the same brand (grain-free) I feed my own girl and the obese cat is again jumping for "da bird" so he figures she's lost some weight already. Both of his fat cats did some time on the streets of DC so there are probably behavioral issues regarding food. Obese cat's daughter was born in a shelter and she is normal weight.