actually,
my idea of how the modular system of the ships works - its not the ships volume that limits the number of modules,
its the ships computer that has a limited number of switchboard slots to connect your modules. (like your mainboards number of PCE slots)
even a cargo rack needs a controller, due to high-speed conveyor system that makes them able to almost instantly flush a selected canister out of the cargo bay door.
also, ships with SLF bays obviously do have the BAY installed in to the frame
- what you switch out and occupies possible internal cargo space, is the fighter storage and pre-assembly system.
thats the reason why you can't put two smaller modules in the space of the bigger one (like in an omnimech

)