Blue brindle pitbull??// whys she so small?

So I asked this but I have a five month old blue brindle pit she's very small about twenty pounds every feature of a pit except upward tail blue grey eyes fully brindle. How big are blue brindles suppsed to be at 5 months?? She's from the pitbull rescue I've seen her siblings. They are triple her size. She is the runt granted. Could she be a similiar breed? Like pocket pit?? Im very confused help. Possible breeds??