For the last 40 years my wife and I have had over 25 feline companions. All of them have had access to exactly the same food sources. Over that time half a dozen of them have ended up being significantly overweight. The others have maintained a 'normal' weight in the same environment. Very early on our vet told us that he thought if we wanted to drive ourselves crazy worrying about our cats' weight we were welcome to do so but that in his opinion fat cats more often than not lived as long and were as healthy as their thinner counterparts. After 40 years the results at my house have supported that vet's opinion.