How big are female blue brindle pitbulls supposed to be at 5months?

I have a five month old brindle she's 21 pounds and still super small her feet are small, and far apart is she maybe a pocket pitbull. Her ears go back like a gremlin and her tail is curled. Is something wrong with her? She is a runt. Any suggestions about what she may be? Blue grey eyes pure brindle gremlen ears feet kinda far apart. HELPPPP