Fat to Muscle Ratio.
The adult Chihuahua should have a lean and sleek appearance. With this said, there are many that are underweight and some that are overweight. When body frame size is put aside, the muscle to fat ratio will come into play in the same way that it does for humans.
Chihuahuas That Fall Outside of the Expected Range
A Chihuahua will generally weigh between 2 and 6 pounds (.9 kg-2.7 kg) depending on body structure. One must remember that 6 lbs. maximum applies to show dogs and therefore pets that weigh a bit more than this guideline states is not necessarily a negative thing.
Conversely, weighing less than expected can make for a fragile dog that is prone to both injury and medical issues.
Oversized Chihuahua -
There are plenty of larger, solidly structured Chihuahua dogs that are perfectly healthy. There are also those that are carrying too much fat and are overweight in an unhealthy way.
A Chihuahua may be bigger than expected for one of several reasons:
1) Purposeful Breeding.
A breeder who has experienced the loss that often comes with breeding very tiny dogs may have decided to aim for the higher end of the spectrum; thus producing dogs that have better overall health and are not as susceptible to injury and health problem that plague tiny dogs.
Often the goal is to produce pet quality dogs for loving homes. Not being bred for show, the higher weight is acceptable and allows owners to bypass the somewhat treacherous path of caring for an extremely small dog.
Many owners who have had big Chihuahua and are looking for a second dog will actively seek out a larger-framed Chihuahua. Families find that 6, 7 or 8 pound Chi are more appropriate for families with young children.
Even with careful pairing of two Chihuahua that fall in the breed standard's weight range, there can be an unexpected 'throwback' puppy. This occurs when genes skip back many generations.
The puppy will not have the physical traits of his parents; he will instead take after great or great-great grandparents. Somewhere in the bloodline there may have been a heavier, larger dog whose genes pop up now and again.
3) Technically Overweight.
No matter the body frame, a Chihuahua can tend to gain weight as he ages. This breed is considered to be a senior at the age of 8 years old. With a life span
of 14 to 18 years, this leaves may years of senior living that can affect the weight of the dog.