The original Yorkies were actually quite small. Recently, as we celebrated veterans day, I saw posts about, the now famous, war hero, Smoky, the Yorkshire Terrier who fought is WWII. He was so small he fit into his new owners helmet even though he was full grown. He was 4lbs and about as rugged as a dog can be. If you can find his life story it is worth the read time. I know he/she? has had a lot of press over the years.
https://www.facebook.com/smokywardog
These dogs had a much wider gene pool I'm sure and their size was not achieved by the bad breeding practices used today in order to downsize a breed.