I think you need to learn more about how databases work. They don’t typically reclaim deleted space automatically for performance reasons. Databases like to write to a single large file they can then index into. Re-writing those files is expensive so left to the DBA (you) to determine when it should be done.
And how are you backing up the database? Just backing up /var/lib/postgres? Or are you doing a pg_dump? If the former then it’s possible your backups won’t be coherent if you haven’t stopped your database and it will contain that full history of deleted stuff. pg_dump would give you just the current data in a way that will apply properly to a new database should you need to restore
You can also consider your backup retention policy. How many backups do you need for how long?