Adding to point 2. Although Capacitor/Battery banks (China doesn't use Compulsator) take space, the conventional ammunition magazine takes much larger space because of the propellant casing while railgun magazine only store the warheads and the warheads are much smaller than conventional counter part because the lack of explosive charges. In the end physical volume of railgun may be the same or smaller (if the capacitor further reduces size in future development).Machinery/fuel spaces to generate power basically require two elements ---
1)DC power source/AC + alternator - This would likely be provided by IEPS on a non prototype ship like the type 055A/052E(?), so no extra space required local to firing system,
whereas in the current prototype 072,we can expect the areas that normally hold vehicles to have been converted into areas containing extra electricity generation systems (diesel generators/auxillary power)
2) Capacitors/ Compulsator system - This is probably what needs space to be placed locally to firing system, however the arrangement of this system can be quite flexible as it isnt a single item with rigid dimensions but a series of components that can be connected in a loosely coupled manner.
In other words it can be integrated close to the reload system with flexible constraints on required dimensions.
Also i believe the meaning of freeing up space isnt literally in terms of de-allocating physical space but notion of realloting the vls to different roles.