Many people like to refer to small dogs as "teacups", "minis" or "micro puppies". There is no "teacup" breed of dog and neither is there a specific differentiation between a "teacup" yorkshire terrier or maltese and a toy or regular yorkshire terrier or maltese. All yorkie and maltese are toy breeds.
"Teacup" is a fictitious term used to describe puppies that are under a certain normal weight for the breed standard. For example; many people use the term "teacup" to describe a yorkie or maltese puppy that is under 4 lbs fully grown. The breed standard for maltese and yorkies, as described by the AKC standards, is that the dog must not exceed 7 lbs at maturity. However, many breeders would agree that a yorkie or maltese that is under 4lbs full grown at an adult age, usually has many health problems (hereditary and non hereditary) more so than a dog that is between 4-7 lbs. Pups like these are usually individually referred to as "runts of the litter". Birth defects and organ failure are common issues attributed to these runts. Breeders would also agree that there is no way to intentionally breed these puppies to be under 4 lbs fully grown unless you bred two "runts" together and thereby perpetuate the bad genes of the runts and encouraging bad breeding. A reputable breeder would not breed for a smaller size puppy but rather to perpetuate good genes in order to keep up the quality of their breeding stock and to produce puppies of good temperament that conform to the breed standards, aesthetically and sociably.
Of course, this is not to say that every puppy under 4lbs at maturity will be unhealthy. There are sometimes one or two pups in a litter from two healthy parents that just happen to be smaller in size. And thus, just like humans, this creates a little diversity within the same breed of dog.
When looking for a small puppy such as a maltese or yorkie, it is imperative that you consider temperament as a major factor before considering the size of the puppy. Size may matter to a certain extent depending on your situation and a reputable breeder/handler would be able to give you a close estimate as to the pup's adult weight based on his/her weight at the time.