Medium and heavy mass drivers (TacOps) are allowed to exceed the 70-point bay limits because, well, they do 100- and 140 capital points per shot.
However, their very limited firing arc, +2 to-hit penalty, and ridiculous tonnages (50,000 and 100,000 tons) mean they're pretty useless compared to any other combination of bays adding up to 140 points of damage.