The reason yorkies come in so many sizes is because when the yorkie was first bred it was a much larger breed and was bred down in size over the years by breeding 2 runts of litters together to produce a smaller breed so even now yorkies of the same litter can be different sizes.
Years ago what is now called a 'tea cup' would of just been called the runt of the litter which most people did not want as it was associated with health problems so as you can imagine breeding two runts together was prob not a good idea and could be where the breeds health problems come from today.
Breed standard of a yorkie is up to 7 pounds but thats just for showing yorkies.
I have had yorkies from 6 pounds up to 18 pounds.
