As far as I can see, the modules are limited in that way due to the availability of internal power counduits, data cabling and conveyor systems. Splitting a module wouldn't simply be a matter of putting two smaller modules into the same space, as you would also need to introduce the extra power distribution systems, network bridges and conveyor splitters necessary to keep both modules fully integrated into the ship systems.
Originally Posted by
Stealthie
(Source)
The main "issue" is that module categories increase linearly whereas module sizes/capacities increase exponentially.
It's not an insurmountable issue but it's one that would probably require quite a bit of thinking about to ensure it doesn't create game-breaking imbalances.
For example...
A C2 cargo rack can hold 4t of stuff.
A C5 cargo rack can hold 32t of stuff.
Ergo, a C5 slot can accommodate 8x as much stuff as a C2.
A C5 HRP adds 390 integrity.
A C2 HRP adds 190 integrity.
With this idea implemented, I'd be able to stick 8x C2 HRPs into a C5 slot for a gain of 1520 integrity.
While that is a good point, it's also worth bearing in mind that hull tanking tends to be pretty strong on smaller ships but pretty useless on larger ones, perhaps this is what is needed to bring hull tanking on the big three into line with the current SCB stacking? Similarly, trading scales pretty hard with increased ship sizes, yet other activities such as mining are limited by the exponential increase in controller sizes for a linear increase in performance. In fact, the borked scaling on many internal modules are what severely limits their potential as a solid endgame profession, as it's impossible to balance both beginning and endgame loadouts if some things scale exponentially while others scale linearly (balance them for small ships and they'll be useless for late-game earning, balance them for endgame loadouts and they'll be horrifically broken on a sidewinder or cobra). I'd say that, splitters or no splitters, FD do need to take a good look at the relative scalings of their modules.