Abnormal Small Golden Retriver

Question
QUESTION: I just purchased a 6 month old Golden from a local breeder.  He said she was a leftover from his last litter. I thought she was a bit small (he told me she was 4 mo but the AKC paper said 6) and I immediately took her to the vet.  The vet said she should be 3 times larger than she was at this age.  He said we may have the world's only miniature golden.  She is not hyperactive (I thought of thyroid problems) an she is healthy but I am concerned there may be a cause for this I am unaware of.  I could have gotten one of his other puppies but i could not just leave her there.  She is about 5 lbs underweight for her current size but he could find no other apparent problems.  If anyone has ever seen this I would appreciate the information.  She is 22 lbs right now but eating very well.

ANSWER: Although purebreds sometimes run small, a 22 pound Golden at 6 months sounds out of the range.  I just had a Lab that was only 50 pounds at a year, and she was bigger than that at 6 months.  I wonder about the breeder, being wrong about the age and under feeding the dog.  She truly is underfed and not just lean?  Your dog definitely should be narrower at the waist than the hips and chest.
You should be able to easily feel the ribs, but not see them.  Each dog is
different.  Standard recommendations are a good place to start, but each dog
must have its food adjusted to its individual needs. Here is a link to a good illustrated guide, http://www.longliveyourdog.com/twoplus/RateYourDog.aspx  Some vets are behind the times and encourage overweight dogs.  Usually it is more of a problem keeping Goldens lean than up to weight.  Eating well is typical.

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QUESTION: She does not really appear underfed.  She is proportionate size to weight but just tiny.  The AKC paperwork says her birthdate was May 6th and the vet said he could tell from her teeth that she is 6 months old.  From what information I can find she would be at around 75% of her adult weight at this age.  If that holds true she would be around 30 lbs or a little bit more as an adult.  She definitely looks like she should only much smaller.  The vet said it was just genetics.  I just wondered if anyone has seen this before or could think of what any other reason for her size could be.  thanks

Answer
I guess she is just a real runt.  At 6 months she should have all of her adult teeth except perhaps the canines.  That 75% may vary.  Dogs aren't very consistent about it, but I wouldn't expect much more that the 30 pounds you suggested.  

I once saw a very small Lab looking dog.  The shelter gave her to somebody as a Lab puppy.  They took her to the vet and were told she was pregnant.