With the Chihuahua breed, the smallest of the litter may end up being the largest adult since growth during the first year can be so staggered.
With this being said, there are far too many breeders that are working diligently to purposefully produce Chihuahuas that are smaller than nature intended.
Let's keep in mind the size of a standard sized Chihuahua:
AKC - No more than 6 lbs. (2.7 kg)
UK - 2 to 4 pounds is preferred (.9 to 1.81 kg)
CKC (Canada) Up to 2.7 kg (6 lb.), with 1.8 - 2.7 kg (4 - 6 lb.) preferred
FCI (Europe) - 3.3 to 6.1 lbs. (1.5 to 3 kg)
As you can see, this is a very small breed and the Chihuahua is in fact the smallest of all purebred dogs in the world. Miniature Chihuahua dogs - when purposely breed to be super tiny - will be smaller than 6 lbs. (2.7 kg) as adults. This gives us adult dogs that are only 2 to 5 lbs. (.9 to 2.26 kg). The puppies will range from just mere ounces to 1 or 2 lbs. (.45 to .9 kg).
It is an understatement to say that these miniature Chihuahuas will have a host of health problems
and will be extremely vulnerable to injury.