I have a 9 month old female APBT/AST cross and she is shaped just like your girl. Leggy and lean. I think my girl favors her APBT side, at least so far. Like others have said, she's not done growing and will continue to fill out for the next couple of years yet. It is normal for adolescent dogs to be more lean than normal. I don't think your girl is mixed with anything, she looks all pit bull type to me, she's very pretty. Below is a recent shot of my girl, Chassis, for comparison. More in my profile album, if interested.
A couple of good threads to read thru: http://www.pitbull-chat.com/showthre...-pit-bull-quot http://www.pitbull-chat.com/showthre...e-vs-Phenotype! A thread from another forum with Diesel...he is leggy and was just a bit heavier at 9 months...now 77.7 pounds everyday, all day http://www.dogforums.com/general-dog...anted-say.html
She has a shallow stop and long muzzle. It's cute, don't get me wrong... but it's interesting!