In the official AKC standard of the Chihuahua, a “well rounded “apple dome” skull,” appears in the section on the head. In more vivid terms, a correct Chihuahua head is rounded at the stop around to the ears and between the ears on top of the skull. A picture, however, is shorthand for why this kind of head is described as an “apple dome” skull which you see in the overlay.
Such a head, however, is a serious fault in other toy breed such as the Toy Fox Terrier, and a fault in the Keeshond.
Some standards don’t make an apple head a disqualification or fault, but they do allude to such a head as being undesirable: The Pug standard writes that “the head is large, massive, round – not apple-headed, with no indentation of the skull.” The Brittany “should never be ‘apple-headed;'” in the Samoyed, the skull is “wedge-shaped, broad, slightly crowned, not round or apple- headed,” and in the Lhasa Apso, the skull is “not quite flat, but not domed or apple-shaped.”
There is no one reason as to “why” such a head is a fault or undesired in all but one AKC breed (as of this post), but we can offer a few insights gleaned from owners in other breeds. In some small breeds, an apple head is possibly associated with increased risk for hydrocephalus if its appearance is the result of buildup of cerebrospinal fluid in the skull which causes the top of the skull to become dome-shaped.
In other breeds, such as the American Hairless Terrier, an AHT with an obvious apple head hints that a different breed may be in that dog’s genetic background, and thus, that dog may not be a purebred AHT. This type of head is also at odds with breed type because the standard calls for a skull that is broad and only slightly domed, one that when viewed from the side has a skull and muzzle of equal length joined by a moderate stop.
Images: Chihuahua in overlay by Caterinarufo is in the public domain; American Hairless Terrier by S Author Sergey Tikhomirov/Depositphotos