Hate to say it, but firstly.. There's no such thing as an "indoor cat"..!! All healthy normal cats like to spend a fair amount of time outdoors, and even in really foul weather they will usually want to nose around the garden for half-an-hour a day before coming in again. I do draw the line, though, at the common and irritating misconception that every cat HAS to go out at night, all night - that's just plain ignorance and selfishness on the part of owners who can't be bothered to buy an overnight litter-tray). Secondly, Yes, unfortunately it's very common for house cats to become obese, like many people and for exactly the same reason - too much food, too little exercise. If your cat isn't even interested in looking outdoors, it's almost certainly because it's way overweight for it's size and just can't be bothered to pull its weight about, which is no condition for a happy cat to be in. Cut down the food and especially any snacks drastically, and enforce at least an hour a day every day in the garden during daylight hours, unless the weather is really bad. And even in really bad weather most cats will quickly find a convenient shelter. As a result, your cat will get used to fresh air and walking again, and begin to come alive, and the fatty lumps (and any other excess weight) should then start to disappear.