Given the crap that barely passes for industry pet "food" it's hard to keep an indoor animal slim (it's easier for dogs though since us can openers take them out for walks regularly). Pet foods are made to look and smell appealing primary to us can opener humans, not the pets. And dry foods are even worse for cats and dogs - they are used to stuff like grain as small part of the mice and other rodents they naturally eat, not as filler material.
PSA: Get your pets high quality food that is similar in composition to what they'd eat in nature and not highly processed crap. You don't diet on frozen pizza, burgers and full sugar soda all year, so please don't feed your pets the equivalent.
And you are encouraged to take your cats on walks. My friend runs a line across his backyard and attaches the cat's harness to it. She gets to run around and the bell makes birds very aware of her presence. Telling people to keep them inside doesn't mean you don't play with them (I explicitly said the opposite). I should refine that you shouldn't let them roam outside. On a leash is fine.
People don't realize this, but you can in fact train cats. My cat does tricks.
And if they are indoor make sure they have areas to run around, be sure to play with them (they are a pet! They are your responsibility!), and watch their diet (often people let them get too fat and it takes awhile for cats to lose weight).