For some weapons, size and mount limitations are part of the balancing. So you're never likely to get bigger railguns, or smaller plasmas, or gimballed/turreted mounts for these.
I suppose one way of balancing a big railgun would be to ensure that the weapon takes damage from every shot fired, due to recoil. And when integrity reaches 0%, it gets blown clean off the hardpoint. Then you can't repair it, either in the field or at a station: it's gone, so you'll need to buy another one.