Know what you are talking about. Like the main gun on Geth ship in ME. Like this idea, but it would raise some balance issues, as even a Cutter is mobile enough to use railguns effectively.
Until, and if ED would survive long enough so it would not evolve further without adding player controlled and/or destructible capitals, there is no real reason for such firepower.
And thing is, all current C4 hardpoints are pretty much that as they are. They take a significant amount of hull space even when retracted. They are really huge. If you look even at Vulture, you can easily compare sizes of a pilot chair and a single C3 plasma.
(and seriously, all of 200 maximum economy passengers on a Conda can fit inside a single barrel of C4 plasma, along with their capsule cabins. Scale in this regard is messed up seriously).
I am following X3 space ship classification by duckling syndrome, and Egosoft had clearly taken some clues from previous Elite games, so it should be fair to use it here.
Some medium and all large combat ships are fit into Corvette class, or M6. Those ships act like heavy fighters towards anything of the same size or larger and act as small capitals towards anything smaller. They are not a subject for having some weaponry like that. They are relatively small ships by design and purpose. They have to be at least twice as large to be a subject for having some of such weaponry and give up on any ambitions for fighter class capabilities