My pit is just over a year old and she was the skinniest pit bull and looked like a cow for a long time, but all of a sudden about a month ago ( 15 mo old) she started filling out and looking like a pit bull finally.  I thought the same thing, wondering if she would ever fill out, but yours will in time.  I think females take longer to look muscular than males too.  Currently Lexi is about 60 pounds.