This is what one breeder has on their site:
Special circumstances often come with extra tiny dogs. They are extremely susceptible to both hereditary and non-hereditary health problems, including birth defects that may go undetected for a long time. Other common problems may include, but are not limited to, diarrhea, vomiting, along with extra and expensive tests prior to routine teeth cleanings and surgeries. Small ones are more likely to have poor reactions to anesthesia and die from it. Tiny dogs are more easily injured by falls, being stepped on and being attacked by other dogs. These health problems nearly always result in
large veterinary bills.
Please take this into consideration and make purchasing a healthy pet your top priority, not size. The “novelty” is certainly not worth the pain, heartbreak, or extra expense. Remember, all Yorkies are comparatively small. The most important thing is finding a healthy puppy that will grow into a healthy adult, especially since you looking at an 11 to 15 year commitment with your Yorkie.
http://www.dovercastleyorkshireterriers.com/info.htm