My two cats are very different in size. Mac is small, about 7 pounds, but he covers every inch of ground he stands on! Ming is huge for a Meezer, he's about 20 lbs. I call him Waddles as a nickname, or Fat Albert! Mac is Alpha Cat and runs things. He can back Ming down with a mere lift of his paw. It's funny how they work things out, and your cats have worked things out. As long as they both seem healthy, don't worry too much about who eats what. Diet for the chunky one? I have tried all the diet dry foods and Ming is still big.