It's just not a realistic expectation.
If we ever get to wander around inside our ships, they're going to have to dream up stuff which will fill most of the internal space and then it'll be a case of having "empty" areas which will be populated with stuff when we fit Shields, Cargo Racks, Cabins, SCBs or whatever.
Here's a little sketch I made which provides an idea of how the internal slots of a Cobra 3 would impact on the internal space.
Presumably, most of the internal space is taken up with core systems and there's only a small percentage turned over to optional modules.
Yes and we dont know how big the "Core internals" nor what else is taken up by Crew quarters etc