Look at vacumn command. Also dbeaver is a teally good gui tool. Right now there is one table thats the main cause (i think its called activity but dont quote me on that there was a post about it)
That table is mainly for debugging as it logs every action in activitypub your instance does. I truncate it once a week and vacumn it (shrinks the disk space used). Caution. You must shut down lemmy before doing so. I run containers so i stop all except postgres and clean it up.