When it comes to weight gain, a cat’s body works in a very similar way to the human body: more food equals more fat. Simply put, the more food your cat eats, the more weight he will gain. A lack of exercise also contributes to weight gain. Some cats can sleep a whopping 18 hours per day, leaving only 6 hours for any type of exercise. Cats that are in their senior years are particularly prone to weight gain, as their metabolism slows down and their activity level steadily declines.