A cat will become fat when it is not exercised enough and is not fed the proper amount or type of food. A fat cat isn't necessarily a good thing and obesity in cats can lead to detrimental effects on a cat's body.
In order for a cat to remain at its healthiest, it should be fed a diet that is in conjunction with its weight and breed. Adult cats should never be fed kitten food because kitten food has more fat and nutrients in it than adult cats need. Cats should also not be fed table scraps because it can make the cat gain weight and can cause intestinal problems for felines.
An important aspect that many people overlook when taking care of cats is an exercise routine. Some people believe that simply letting their cat run around the house is enough exercise to keep them fit, but it is not. The best form of exercise for indoor cats is playtime. Cats will not get too fat if they are allowed to play constantly. They should be given toys that promote exercise and should be played with by their owners. A high energy kitten can also encourage cats to play more often.