Pomeranians typicallly do not go larger than 3-7 pounds when you purchase a good quality dog from a show breeder. You can see the breed standard here
http://www.akc.org/breeds/pomeranian/
However the breed used to be about 30 pounds historically, ie small but not tiny, and you do get dogs that are larger sized in the breed if you are not very careful in your breeding practices or if a surprise set of genes shows up in combination in a puppy.
You did pay a lot of money for the pup but depending on where you purchased the dog and what the ancestors were like, and how big the pup was at purchase time having one go this large may not be unusual.
I'd expect pet store poms, or poms from higher volume breeders, to frequently be very large for their breed, as larger dogs produce more pups which increases profits, and they also survive better in the conditions most pet store pups are raised in.
So your dog may be a purebred but be oversized for the breed. If you didn't meet the parents it may be they were large too.
You can try a DNA test to see if she is the offspring of her parents.
If you have AKC papers on her you might work through them to see if the sire and dam are DNA tested to make sure the listed parents are the actual parents.
http://www.akc.org/news/index.cfm?article_id=1778
For future reference a pup at 16 weeks of age is typically at about 50% of the adult weight at maturity.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics
You can go back where you purchased her and lodge a complaint and see if they will resolve the issue.
If you have AKC papers you can ask AKC for help with this.
If you want to just run a DNA test to see if she is a mix or not here is a company that does it
http://www.dog-dna.com/
but I'm not 100% sure on how accurate any of those tests are.
With the AKC DNA test if the parents are on record with their DNA you can check to see if the parents on her papers are her real parents.
Best option if behavior issues are the biggest negative for the dog is training. www.apdt.com may help you find a local trainer to work with or ask your vet.
Poms can be highly active and opinionated little dogs.
Thanks for your above accept!