Dog digestion doesn't differ. The fact is, your dog's digestion times are more likely to be influenced by what you feed him than by his size. Because your small dog eats an amount of food proportionate to his size, his body breaks down food in the same manner as a big dog's would. The time it takes for food to digest in your small dog is essentially the same as in a dog twice his size or larger.
Your dog's digestion rate isn't affected by his size as much as by other factors, including the type of food you give him, the amount of exercise he gets, the amount of water he drinks and more. For example, an average dog -- big or small -- takes between four and six hours to digest canned wet food. Digesting dry kibble, on the other hand, may take between eight and 10 hours. Digestive disorders like irritable bowel syndrome can affect a dog's digestion, too; if you are concerned that your dog is eliminating too often or not often enough, contact your veterinarian as soon as possible -- no matter his size, this is a potentially serious issue.