My 10 month old cat pepper weighs about 10 pounds. He doesn't eat alot of food, as much as a ideal cat would eat. He does drink alot of water. Do you think he is a overweight?
Instead of focusing on his weight, focus on his condition. What I mean is, feel for his ribs. You should be able to feel his ribs easily, under a light cover. If you have to "dig in" to feel the rib bones, he's probably a bit overweight. You shouldn't be able to see the ribs either, if the bones stick out so you can see them, he's probably a bit underweight.
If it is an excessive amount you might want to take him in and check him for diabetes. I am not saying he has this, but if you think he is fat, and he is drinking a lot of water- emphasis on a lot- it is a possibility to check out. Of all the cats I have, two are really overweight and both are because of different health reasons not related to how much food they actually eat.
Tasha, if you can feel his ribs easily, he's probably not overweight. Cats do not have a set ideal weight that fits all cats, no more than people do. Some cats are naturally bigger, some smaller, and they are not the same weight or even close. As long as you can feel his ribs, his backbone and such, he's fine. Look at my cats- the older one (domestic) weighs about 9 lbs, and she's just right, as I can feel her ribs easily. My 10 month old (Maine Coon) weighs about 10 lbs, and she's definitely underweight, since all her bones are way too easily felt, and could be seen if she didn't have a long, thick coat.
But like hissy said, if your cat is drinking heaps of water, he should be checked out by the vet, as this can be a sign of illness. The vet would also be able to tell you whether your cat is overweight or not.