I recall that the original idea of the reduced shot railguns was to make it easier to mount them on fighters, based on the B5-theme campaign Steve was playing at the time using railgun fighters, surprise surprise.
I am not sure that Steve had worked out how mounting single-shot railguns of large calibers could be used to replace full-size railguns to provide higher DPS. As it is you can put out around ~3x the DPS at the cost of only 75% as great of an alpha strike, once you account for necessary power plants to charge so many capacitors. I am split on whether this is a good thing for "game balance" (yes, yes, "Aurora isn't a balanced game"...). On one hand, it gives railguns a very strong niche as high DPS with a close range. On the other hand, I am not sure if it is not too strong to the point where the classic four-shot flavor mechanic of railguns is forgettable by comparison.
If we want to nerf the reduced shot railguns, I think it can be as simple as reducing range, maybe not proportionally but by a square-root rule so that a single-shot railgun has 1/2 the range of a four-shot railgun. I think this would preserve a balance between different sizes as the full-size weapons have lower DPS but longer range and of course alpha strike capability.
Another option would be doing something like what was done with lasers and cutting the recharge rate down proportionally. Unlike lasers, this can simply be proportional i.e. a single-shot railgun has 1/4 the recharge rate as a four-shot railgun, so that DPS is the same in each case. This means that reduced shot railguns only have a niche as a way to mount a large weapon on a small platform (fighter/FAC) which I think was the original intention. However, this does feel very limiting for what is a fun new feature as reduced shot versions are now strictly inferior...
Alternatively we can ask if the balance is fine as-is, and if instead we need to buff reduced-size lasers? Here I am not sure. In my mind, for Aurora "game balance" means not that everything is equal but that everything in the game has a purpose and a niche which is not eclipsed by some other feature or mechanic. In this sense, reduced size lasers do have a niche (put a big, long-range laser on a small fighter or FAC and point them at the enemy). So personally I do not think reduced size lasers need any buffs to remain functional even if they are in most cases unimpressive (admittedly, this is in large part because beam fighters are unimpressive as long as missiles are a thing).
IMO, the railgun balance is okay as it is but I do feel like reduced shot railguns are generally preferable to full-size versions for large calibers. If a change is needed I think the range reduction I proposed as sqrt(size) would be perfectly suitable.