Put your hands on the side of the chest cavity and attempt to feel the ribs. If the ribs can be easily felt, but not seen, the animal is in normal condition. If you have to push hard to feel the ribs, a layer of fat is probably obscuring them. Also, the abdomen of normal cats and dogs should be tucked up when viewed from the side, and they should have a waist when viewed from the top. Cats often develop large fat pads on the abdominal wall between the hind legs, which are sometimes confused by owners for tumors.
Animals become obese for the same reasons people do.
That is, they take in more calories than they burn. Metabolic diseases occasionally result in obesity, but this is a very uncommon problem in Veterinary medicine. Obesity is almost always due to a combination of too much food and not enough exercise. Treatment of obesity consists of a combination of diet and exercise. A number of reduced calorie foods are available and may be recommended for different situations.