Shani (and all others with heavy kitties), I have a cat that was pretty big, too. I did some research on the net and this site seems the best info-wise
http://www.pet-grub.com/preface.pets
Since I have been feeding them raw food, Kuma has lost about 3 kilos and Tabi is about the same as always. No dry food or canned. Their coats are wonderfully healthy and their eyes are very clear. They are energetic, they shit less and it doesn't stink as bad. I would recommend everyone give this a try.