Hi,
Your yorkie may not be a mix.. I don't believe the breeder would be able get registered papers from AKC if that was the case. If your puppy is registered most likely it is a pure breed. What I think has happened is you may have picked the larger puppy in the litter. When we purchase puppies they are for pets unless you specify you want a show dog. The beeder will not sell you a show dog with out a detailed contract. The standard for yorkies I believe is max somewhere from 6- 7 pounds. That is a breed standard.
In my mind that means ,breeder this this is the standard if you want to show your dog and win. So when we purchase pets they will be slightly off the standard. Weather it's the hair quality the eye, the legs being to long, the way the puppy stands up or the way the puppy walk. What I mean by that is something may not be standard quality for the show ring. Does this mean your puppy is less perfect of course not. Your puppy is perfect . Just may not be show quality. Now your vet on the other hand is not perfect and do not let him or her upset you. If you are still concerned, your best contact is your breeder. We have yorkie and a Maltese both from differnt breeders. One breeder that does shows and won best in breed last year. She picked out our Maltese for me... she will be on the bigger side. Yes I could of picked a smaller puppy , but this puppy fits our personality better. I know she is not show quality as she as some lemon on her ears and a little on her back and her hair is more cotton then silky, but she is still a pure breed dog. I saw the parents and they were both beautiful dogs and both show winner and with in the standars. I'm sure there are other here that could add more.. but this is the way I look at it.
I hope this makes you feel better
Here are some details on the standards form AKC:
http://www.akc.org/breeds/yorkshire_terrier/index.cfm
Yorkshire Terrier Breed Standard
Toy Group
General Appearance
That of a long-haired toy terrier whose blue and tan coat is parted on the face and from the base of the skull to the end of the tail and hangs evenly and quite straight down each side of body. The body is neat, compact and well proportioned. The dog's high head carriage and confident manner should give the appearance of vigor and self-importance.
Head
Small and rather flat on top, the skull not too prominent or round, the muzzle not too long, with the bite neither undershot nor overshot and teeth sound. Either scissors bite or level bite is acceptable. The nose is black. Eyes are medium in size and not too prominent; dark in color and sparkling with a sharp, intelligent expression. Eye rims are dark. Ears are small, V-shaped, carried erect and set not too far apart.
Body
Jump Into Agility
Well proportioned and very compact. The back is rather short, the back line level, with height at shoulder the same as at the rump.
Legs and Feet
Forelegs should be straight, elbows neither in nor out. Hind legs straight when viewed from behind, but stifles are moderately bent when viewed from the sides. Feet are round with black toenails. Dewclaws, if any, are generally removed from the hind legs. Dewclaws on the forelegs may be removed.
Tail
Docked to a medium length and carried slightly higher than the level of the back.
Coat
Quality, texture and quantity of coat are of prime importance. Hair is glossy, fine and silky in texture. Coat on the body is moderately long and perfectly straight (not wavy). It may be trimmed to floor length to give ease of movement and a neater appearance, if desired. The fall on the head is long, tied with one bow in center of head or parted in the middle and tied with two bows. Hair on muzzle is very long. Hair should be trimmed short on tips of ears and may be trimmed on feet to give them a neat appearance.
Colors
Puppies are born black and tan and are normally darker in body color, showing an intermingling of black hair in the tan until they are matured. Color of hair on body and richness of tan on head and legs are of prime importance in adult dogs, to which the following color requirements apply:
Blue: Is a dark steel-blue, not a silver-blue and not mingled with fawn, bronzy or black hairs.
Tan: All tan hair is darker at the roots than in the middle, shading to still lighter tan at the tips. There should be no sooty or black hair intermingled with any of the tan.
Color on Body
The blue extends over the body from back of neck to root of tail. Hair on tail is a darker blue, especially at end of tail.
Headfall
A rich golden tan, deeper in color at sides of head, at ear roots and on the muzzle, with ears a deep rich tan. Tan color should not extend down on back of neck.
Chest and Legs
A bright, rich tan, not extending above the elbow on the forelegs nor above the stifle on the hind legs.
Weight
Must not exceed seven pounds.
Approved April 12, 1966