This was Terri Shumsky's view:
What is a Teacup Yorkie? Is that just backyard breeder hype?
Yorkies according to history were always a small dog, but outcrossing in many cases has caused some larger dogs. There is speculation that some "breeders" have used Silky Terriers and registered them as Yorkshire Terriers. This could happen in kennels where they have more than one breed. They have been bred for 50-60 years as a toy dog which averages 4-6 pounds... there are occasionally 2-4 pounders and occasionally 7-9 pounders. Mice have been mice for thousands of years and therefore the size is more predictable.
Most breeders will not breed a female under 4.5 or 5 pounds because to do so would be cruel and probably cause a C-section or other problems with a crowded uterus, so those that are too small to breed are sold as Tinies... (not tea cups and not miniature... just tiny). Also, small males 2-3 pounds, make breeding a little more difficult, though possible, due to the size of the male in comparison to the size of the female, so most breeders settle on a 4-7 pound size as a preference. It is possible therefore, for pups in the same litter to mature anywhere from 3 to 8 pounds in the same litter. The terminology "tea cup" size came about in the 70's when the breed became more popular than the poodle which had three sizes, toy, miniature and standard. Poodle people would differentiate between toy and very tiny poodles by calling them tea cup meaning they could sit in a tea cup and were tiny. As this size differential doesn't exist in Yorkshires, the terminology is incorrect and seems to be used by some breeders to extract a higher price for the puppy as a "tea cup size." It simply means "tiny." I would venture to guess that legitimate, reputable breeders probably come up with 60% between 4-7 pounds at maturity and 20% under 4 pounds and 20% in the 7-9 pound range. Weight is a consideration but depending on the muscle and bone in a particular dog, they can weigh more or less than you think, i.e. a 12 inch at shoulders dog could be 7 lbs and be very skinny and fine boned, while a 9.5 inch at shoulders could also be 7 lbs. and look much smaller because it carries more muscle and bone. For more information on weight history go to
http://www.geocities.com/~dugmore/weighthistory.html
I think that she and her husband finished something like 37 homebred champions in their lives. They are both gone now.