It's still a prototype, but its a prototype for a different purpose. People use different languages to refer to this. Personally, I call it "iterative prototyping". Others might call it moving from a "works like" prototype to a "looks like/works like" prototype, e.g., http://umaine.edu/amc/2013/08/07/what-is-a-prototype
Regardless of what you call it, you should at all times remain aware of where that prototype lies in your process. My favorite question for students during a design process involving prototyping is "why are you building this prototype?". Obviously, you're building a prototype because you're asking it a question. If you knew the answer to the question you're trying to address with the prototype, then there wouldn't be a need to build it.
If I may wax a bit philosophical, going from small size to large size sometimes shows an idea may not scale well. My absolute favorite example comes from the story of Diognetus in Vitruvius' Ten Books on Architecture. Diognetus was fired from his retainer for the city of Rhodes, because an upstart proposed a scale model for the purpose of defense against siege engines, nabbing them by crane. When attacked by the Helepolis, the siege engine to end all siege engines, the "breaker of cities", he couldn't build it, because it didn't scale well.
They managed to rehire Diognetus, at his own price! (http://lexundria.com/vitr/10.16/gw)
He at first refused to listen to their entreaties; but when
afterwards the comely virgins and youths, accompanied by the priests,
came to solicit his aid, he consented, on condition that if he
succeeded in taking the machine, it should be his own property. This
being agreed to, he ordered a hole to be made in that part of the wall
opposite to the machine, and gave general as well as particular
notices to the inhabitants, to throw on the other side of the hole,
through channels made for the purpose, all the water, filth, and mud,
that could be procured. These being, during the night, discharged
through the hole in great abundance, on the following day, when the
helepolis was advanced towards the wall, it sunk in the quagmire thus
created: and Demetrius, finding himself overreached by the sagacity of
Diognetus, drew off his army.
The moral of the story is hold out for your price!