Population gain by other means
Remember the SCI Pittsburgh facility was shut down over the last decade, an event that caused a minor amount of hand wringing at the time, mostly I think because of the impending job loss associated with the closing. Then with prison overcrowding in Pennsylvania it was reopened. According to the story it now has 1,600 residents. There have been news stories over population movements of a few thousand people here or there.
We just had a census right? and some don't realize this, but prisoners generally get counted where they are incarcerated. All 1,600 of those folks are for census people-counting purposes, residents of the city of Pittsburgh.. as are the inmates at the county jail for that matter. I don't have an official number right now, but the county jail has been near its 2,500 capacity for years. So between the two institutions you are talking over 4K people, which in itself is over 1% of the city population. It's enough to matter to things like population trends, tax collection, school enrollment, etc.
Implications. These folks all count for redistricting as well. 1% may not be a lot, but it will impact some local political borders a lot. Those folks all count for local districts as well. So prisoners could easily represent several percent of a state general assembly district. The co-located 2,500 county jail inmates will make up roughtly 8% of a city council district after the next redistricting. They may not vote (much.. most are indeed legally allowed to vote), but they count when it comes to drawing district boundaries. Whether anyone has ever formally or informally campaigned inside one of these institutions... I dunno?