When my parents bought our "yorkie" 9 years ago, they were shown his parents - two adorable pint size tea cups yorkshire terriers. They were told he would be their size when he got older. Our "tea cup" is more a latte mug.
Over the years people have plainly told us he's a silky not a yorkshire terrier. A silky is a mix of Australian Terrier and Yorkshire Terrier. They tend to be larger in size, though they are still a small breed. We (family, vet, random strangers, silky and yorkie breeders) took a vote ... he's a silky.
Random yes ... important? Not really. Breed isn't important, it never was, even now it's not important. It is a little helpful when I tell people about him because I get lectured when he's a 11 lb 9 oz yorkie.