A reduction in caloric intake is necessary for an overweight cat to lose weight and, if possible, an increase in activity. You can start by reducing the quantity of your cat's food by one quarter. It may also help to divide your cat's food into several small meals given throughout the day if he seems overly hungry once you've decreased the amount. Also be aware of the amount of treats you're giving to your cat on a given day and try decreasing these as well.