Chihuahuas pack a lot into a tiny body. They are intelligent, require little exercise and grooming and can live quite happily as city dogs. And, as small dogs, extra weight can be detrimental to a Chihuahua's joints.
According to the American Kennel Club, Chihuahuas should not exceed 6 pounds. Doing so puts extra stress on their joints. A veterinarian can help monitor Fido's weight, ensuring he is not exceeding what is recommended for his size.