Bet you anything they are just 2X6's. Over time they have sagged. They are still holding the walls in and the ceiling up and likely will for another 100 years if the house is maintained.
Only way to fix it would be to gut it and sister in better sized joists, new subfloor, and then re-sheetrock it all. Technically a 2X6 can span just over 9' as a floor joist. Granted if the house is old enough those joists are thicker and more like a full nominal 2"x6" so they are certainly better than todays crap pine.
Likely will be fine, just don't put a pool table up there lol