According the AKC guidelines a Chihuahua should have:
Head: A well rounded "apple dome" skull, with or without molera. Muzzle – Moderately short, slightly pointed. Cheeks and jaws lean. Stop - Well defined (the stop is the angle of change between the skull and the nasal bone near the eyes).
Take note of the term “moderately short” in regard to muzzle. Appleheads have shorter muzzles, while deers have much longer muzzles.
The CKC (Canadian Kennel Club) calls out for the exact same physical traits:
Well-rounded apple-dome skull, with or without molera. Cheeks and jaws lean. Muzzle moderately short,
The UK (Kennel Club of the United Kingdom) calls out for a very similar appearance:
Well rounded 'apple dome' skull, cheeks and jaws lean, muzzle moderately short, slightly pointed. Definite stop.
The FCI ( (Fédération Cynologique Internationale - International kennel club with 84 member countries) defines the face and skull shape in this way:
Skull : Well rounded apple head (a characteristic o f the breed) Stop : Well marked, deep and broad as the forehead is rounded above the set-on of muzzle.
There are many Chihuahuas that fall somewhere in the middle between the 2 types. Each dog is an individual...he or she will vary in size, body structure, weight, etc...And the exact shape of the head will vary as well.