Do you have Archiving active?
This creates a new archive of your server(s) and all files assosiated each time a new archive is produced. Look in /var/games/minecraft/archives/. there you will find folders for each server containing the spesific servers archives. Delete the older ones of the files unless you need to roll back very far.
As for adding drive space:
Adding an USB HDD: https://linuxconfig.org/howto-mount-usb-drive-in-linux
You can also mount a NAS: https://linuxconfig.org/how-to-configure-nfs-on-linux
Or add a windows share: http://www.linuxnix.com/8-ways-to-mount-smbfs-samba-file-system-in-linux/
(Be adviced, CIFS have a problem with how the archive filenames are constructed. It do not like “:” in the names, so you’ll also have to edit a mineos-file to be able to use cifs/samba: Mount NFS share for archive/backup)
After this you need to do tow things: move all the files you want stored elsewhere to the new mount, and symlink the new position to where the old one was (or MineOS won’t be able to use it).