As discussed in the comments on Realistic Population here is the save file where there are over 700,000 citizen units being used by the Airline Head Quarters Building. You should be able to confirm this by deleting the building and you will then see the Citizen units used drop.

I'm finding that the Citizen Units grows at around 50,000 per hour played. I can keep it under control at the minute by periodically deleting and then undoing the building.

https://steamproxy.net/sharedfiles/filedetails/?id=2930384608

Let me know if there is anything I can do to assist in troubleshooting.