She is too big for a bantam. It is mainly her lack of maturity that has puzzled me more than her size, even though she is much smaller than she should be.
My BR is also a lot smaller than I think she should be. She's active, healthy, and eats well, so I'm not overly concerned, but I did just start a round of Rooster Booster just in case it's worms that are slowing down her growth.