Scaling up a small drawing to a large one is no problem to people with such skills as the Iron Age people had. If you can work out intricate geometric patterns that fit exactly around a stone etc then you can scale up a few curves. As Uffington isn't properly visible from below it couldn't have been done by a bloke at the bottom of the hill. (I also remember reading that Uffington has changed over the years - or is that somewhere else?)