Don't know if I am posting this in the right place, but is there a way to determine a kitten's age? The paperwork says 9 months, but I think she is much younger. I have posted pics under the furr and pics section in the post "name suggestions".
I believe that in kittens, the teeth can give a pretty good estimate of age.
I saw your other post, and I see that your girl was recently rescued. Is it possible she is small because of poor nutrition in the past? (In any case, I bet she's happy to be in her new home!)
A vet may be able to tell from her teeth. I once adopted what I was told was a 4 month old kitten, who turned out to be about 7 months old and undernourished, according to my vet.