The 6x12" cabinet that came with the 1200S is 30" wide. However, the 612S cabinet is the same size as the 610S, the same size as the 215S and the 312S. It was Sunn's standard cabinet for years. Older cabinets were also that size, including the cabinet for the 200S and many other systems.
The main difference between the 610S and the 612S, besides the size of the transducers, is the baffles. The baffles for the 612S are larger, to accommodate the larger transducers, so the angle created by the 612S baffles is smaller. As a result, the 612S has less internal volume than the 610S, and less bass response than the 610S.
If you want to enlarge the holes in the baffles to accommodate 12" drivers, and there's room, well, it's your cabinet.