When postgres deletes a row, it marks a disk space unused. If you delete a lot of rows, there will be plenty of unused space, but that space will not be release

When postgres deletes a row, it marks a disk space unused. If you delete a lot of rows, there will be plenty of unused space, but that space will not be released to OS. To release that space to OS, you need to run vacuum full or perform backup/truncate/restore. In normal circumstances it's not needed. The unused space is reused for further inserts and if your database maintains steady rate of inserts/deletes, this won't be an issue. However I experienced situation when database starts to grow uncontrollably and it was happening for weeks (when actual workload didn't change). I don't know what causes that behaviour. The solution was to run `vacuum full`.