What kind of food do you feed them?
I feed my cats 1/8th of a cup each per meal, 2 meals a day (1/4 each). Initially I did it so they would lose a little weight (they weren't much overweight but even a little is too much), and now I keep it up as maintenance. I was feeding them Iams, but when I get another job at a clinic, I think I will change them to Hill's. At the moment I'm feeding horrible supermarket food, but they enjoy it and are still happy, healthy and active. Their weights are very steady.

I know it seems like a small amount (1/4 cup each/day!), but it's enough for them, and they're pretty active inside-outside cats.
Also, I have a Hill's measuring cup that I have cut down to the 1/8th mark, so now I just get enough food to fill the scoop and no more. It's a good idea getting one if you don't have one - any clinic should be able to give you one if they have them in stock!