Some dogs are small because they are small, some are small because they are unhealthy and not growing as they should. If you go to a breeder who is reputable and breeding quality Yorkies, eventually they will have a small one. Any dog's health depends a lot on it's parents and it's breeder...more than it's size. I have a small Yorkie who is very healthy. Her shots and her spaying was a breeze.
If you get a smaller Yorkie they do require a little more supervision and awareness. You have to make sure no one steps on them, make sure they eat, make sure they don't jump off of high places, make sure they don't run with little kids that can trip on them etc. For me, it is worth the extra effort at this stage of my life but they are not for everyone.
ALL Yorkies are Toys, some just come smaller than others (and some bigger). The word teacup is just used by some breeders to make you think you are getting a special model
. Many end up larger than they say and most of the breeders that use that term are not breeding for quality. Only size.