Whenever I see a cat that's overweight, I have to admit that I'm more likely to blame the owner than the actual animal itself for just being greedy for example. Cats are usually quite well disciplined when it comes to food and exercise, and they'll usually only eat until they are full, and they won't continue to carry on eating till the bowl is empty and then beg for more like dog's do. Indoor cat's are more prone to being overweight as they're obviously not getting the same amount of exercise as their outdoor counterparts. With that in mind, it's important to make sure that you're able to play with them more than you would with outdoor cats to make sure they are getting the exercise they need.
Checking the fat content and the nutritional information of the food is important also, and these days you can even buy cat food that's designed specifically for indoor cats, and while I've never used it so I couldn't tell you how good it is, if you're concerned about your cats weight then I'd suggest its defiantly worth a try as it is supposed to be a lot lower calories then the standard cat food.
