Well it is hard to say as many dogs get a growth spurt later, some early, all depending on the line, the number of puppies in the actual litter, and so on. So the answer is this is not necessarily a concern, as long as your vet says she is fine and healthy. And she my well reach breed standard anyway, she is still very young. and that isn't all that small, either, just slightly smaller. Girls are usually about half their adult size at around 14 weeks but this is just a rough gauge and there are many variables (eg puppies from large litters may start off a lot smaller).
Have you discussed your concerns with her breeder? Any reputable, health focused breeder will only aim to breed dogs that fit breed conformation -- 13-18 lbs in the US, 12-18lbs in the UK -- but some will always be a bit smaller and a bit larger. Talking to your breeder should reassure you of the size of parents and whether her size should be of any concern. Also it may be useful to the breeder to know that she remains small.
Unfortunately there are also many unscrupulous breeders who do not care about what the breed looks like or whether they breed for health, and some even deliberately breed for extra small dogs (eg teacup cavaliers -- there's no such thing). If puppies are well off conformation (a lot smaller or a lot larger), and you don't know much about the breeder, chances are they also did nothing to ensure breed health and just bred any two dogs, in which case there could be longer term implications -- so that is one possible worry with an extra small dog. Breeders who deliberately breed small dogs are really dicing in a cruel way with genetics. First off if they are breeding in this way you can be sure they do no health testing at all and do not understand the genetic problems they can be introducing.
Second they may well be selecting dogs that are already health compromised as that may be producing the smaller dog to begin with -- open fontanels, for example, or liver misfunction... or breeding litter runts, which are usually pretty poor breed examples and often are small due to poorer health than siblings.
And that's the last possible concern -- that an actual physical problem may be the cause of a dog not reaching normal size. However, if the dog is otherwise healthy and your vet isn't concerned, then an immediate physical problem is really probably not something to be concerned about.