The growth rate varies greatly among pups. Some are finished growing by 6-8 months, while the biggest majority may grow until around 18 months. Others may grow until around two years. Surely you aren't concerned about your tiny pup gaining much weight, because I don't believe there is a very big chance of that.
You might also be interested in viewing the following pictures of yorkies from one day old to sixteen years old:
http://www.ytca.org/pictures.html