Many of the game's "core" ship designs utilize the fact that a ship is not required to actually display the components it uses--the 'looks' and the 'works' of the ship are defined in two separate files. So the Battle Hammer does indeed actually use a small hull as its model, while being treated as if it has a medium hull in the game.

The only way to use it as another size would be to mod the game or manually change the ship files after creating a ship. Modding is the easy route, as it's already been done--see my hull mod on the library, which lets you use any model as any size and even combine multiple hull models into one, or use none at all and just build ships out of the 'extra' pieces.