My take was that much of DS9 was unused. It was originally both an ore refing facility and a military base. After the federation took over there was basically no use for much of the ore refing facility. There must have been large storerooms for both raw and processed ore. Along with large ore handling systems to move bulk ore.

I bet its ability to dock multiple capital ships was probably adapted from being able to dock large bulk cargo ships for the ore. Think about it like current bulk material handling terminals for ocean going cargo ships. They are vast areas with relatively few people working at them.

Additionally I would assume that there were military brackets that were unused. I think it is a safe bet that the Cardassians had a much larger military presence there. Just think of how easily it absorbed the additional personal need to operate as the in theater HQ during the Dominion War. It was able to have a Federation, Klingon and Romulan General Staff along with some kind of unified command. While also serving as a place for RR and ship repair.

And now that I typed this all out, I realized I really need to find something better to do with my life. 😂