You may wonder what the difference is between a 'regular' Chihuahua and a teacup Chihuahua. The truth is, there is no difference at all and there is no such official breed or variance as a miniature or teacup Chihuahua. The Chihuahua is the smallest dog breed of all; he simply cannot get any smaller.
Yet, this term is used quite often. Typically, it is used by breeders as a marketing term. Since the words tiny, teacup and miniature succeed in catching the attention of a throng of puppy buyers, breeders around the world label dogs as such - even if the puppies met established breed standard size.
Of course, misleading the public is unethical and is a gauge of one's moral compass. With that said, there are some reputable breeders who will use the term.
There are 2 reasons for this.
1) The first has to do with competition. Before 1990, it was word of mouth that helped a breeder establish his or her reputation. A person's name would be circulated around the show ring and little if any advertising was needed.
Other, less reputable breeders - puppy mills in particular - would simply sell puppies to pet stores. Once the Humane Society of America and other organizations worked relentlessly to stop the formation and existence of puppy mills, this lead to many pet stores limiting their merchandise to supplies and no longer included puppies that the public was - rightfully in most cases - led to believe originated in mills.
While a breeder may indeed gain a solid reputation in the show ring, the socially driven, website world that we live in today dictates that a business needs to have an online presence. Even if that business is a small at-home breeder.
Keeping this in mind, one aspect of ranking where the potential puppy buyers will find you is by having certain words appear on your site. We do this here; you'll notice that the term 'miniature Chihuahua' appears many times