Professor David Raubenheimer is a world-renowned nutritional ecologist (and a Kiwi). He has developed a model investigating the interaction of macro-nutrients (carbohydrate, fat and protein) in various species, including primates and carnivores.
Raubenheimer has established that if you dilute the protein in a carnivore's diet with extra carbohydrates, they will follow their innate drive to get more protein by over-eating carbohydrates. He has identified this as a significant contributor to obesity in people as well as animals.
Processed cat and dog food is low in protein (relative to what it would be in the wild). So Raubenheimer's model helps explain why so many pets end up over-eating their processed food (trying desperately to get enough protein), and end up overweight. It also explains why so many pet owners find that, once they switch their cat or dog to a raw diet, they no longer have to struggle to regulate their pet's weight.
In Raubenheimer's latest book 'The Nature of Nutrition' he makes the point that when cats fail to eat the correct macro-nutrient balance:
"cats fed commercial foods will be held in a chronic state of nutritional imbalance. This raises welfare concerns, and will also in the long term likely affect cat metabolism and health."
Read more about Raubenheimer's work in humans, dogs, cats and horses.