Just like people, animals have individual metabolic rates that help regulate

whether they maintain average weight, or become overweight.  Some breeds also have

a natural propensity for either a svelte, or heftier shape.  As a group, however,

pets face the same problems of weight gain that their owners do.  Overeating,

snacks and not getting enough exercise are usually to blame.  Dogs and cats are by nature hunters, not grazers.  In the same way the human body does, your pet's body is designed to store energy as fat as precaution against lean times.  If allowed to eat as much as they want, many pets will consume food as if they were out in the wild hunting, eating as much for as possible, when all
they're actually doing is lying around all day.  When we feed them the proper amount of food and exercise them regularly, pets tend not to put on weight.