What is a Commander doing commanding a Space Station?

The Nimitz often has an admiral aboard as well, commanding the entire group. Then there's also a captain or commander who is CO of the air wing.

I don't see any reason to assume the Bajoran Milita to Starfleet ranks are any different than Army to Navy ranks in the U.S. today. Kira is one rank below Sisko at the beginning, two ranks below after he is promoted to Captain, then one rank below when she is promoted to Lt. Col.

 

Theodor Leutwein was a colonel (Oberst) when he was made governor of German South-West Africa, Germany's most lucrative and populous colony, with the largest German settlements, and the one that held the most strategic interest due to its proximity to the southern African colonies. Today in the US, his responsibilities would belong to a general.

Things change. The meaning of ranks change. The understandings of the importance of missions change. The US had no admirals before the 1850s because they saw the rank as an affectation of royalty. Today a commander of a US base would likely be a captain,but that makes sense in our context. Another organization in a different time might make a different decision.

 

As others have said, DS9 was meant to be an arse-end-of-nowhere posting until the wormhole appeared.

Anyone with any sense would have immediately assigned someone of much higher rank to such a strategically important location, but it's a TV show so they didn't. And then later when they made him a Captain they put him in command of massive war fleets in a tiny little fighter ship when making him an admiral and putting him in the DS9 equivalent of the USS Vengeance would be more believable.

They also let former enemy spy Garak (who betrayed them once before) hold a position of import on the Defiant bridge. Because why not?

 

Why not indeed? At the very least, he's good conversation.