Cats are semi-social, and use urine and feces to advertise their dominance-so only the 'lead' cat (or cat that wants to be lead cat) will advertise by spraying or leaving feces in the open. Dogs communicate with smell and have a more social lifestyle. Urinating on an object marks not only where the dog has been, but who the dog is and, to some extent, how healthy the dog is to advertise it's pack as dominant in the area. Cats are (normally) smaller than dogs. A few breeds of dog have been bred for mutations in body size that don't scale every part, so a small dog might have a larger skull, but, from personal observation, that doesn't make them any smarter.