I have been pondering this a bit, but there is little logic that the dorsal mount of a raider can pack as much punch as the dorsal mount of a grand cruiser. One way of resolving this would be:

Massive Space and Power:

The dorsal weapon positions of a battlecruiser and a grand cruiser are so vast and can handle so much power, that they can fit a weapons battery of broadside strength.

The prow mount of a grand cruiser are so vast and can handle so much power, that they can fit a weapons battery of broadside strength.

Obviously, this would rather increase the fighting power of these ships, so might be destabilising. On the other hand, anything the players can do a GM can do and as there is more chance of a GM fielding these ships, the destabilising effect would be limited.

Friedrich van Riebeeck, Navigator Primus, Heart of the Void