If VPOP3 is using a lot of disk space, the problem is usually that there is lots of data there, so it has to use the disk space.
In VPOP3 version 5 and later, all users' messages are stored in the database, so they will occupy space in the VPOP3\pgsql\data folder.
In VPOP3 versions 3 and 4, users' small messages are stored in the database, so will occupy space in the VPOP3\pgsql\data folder. Large messages are stored in the VPOP3\_messages folder tree, so occupy space there.
Note that in VPOP3 v3 and later, deleting messages from users' mailboxes will not necessarily reduce the amount of disk space used by the database. See the database_maintenance topic for more information.
In VPOP3 versions 1 and 2, users' messages are stored in the VPOP3\<username> folder trees, so occupy space there.
If you have message archiving enabled, then the archived messages are stored in the message archive folder (which may be VPOP3\_archive, but can be placed elsewhere)
If you are using the spam filtering in VPOP3, then quarantined messages are stored in the VPOP3\_quarantine folder for the number of days specified in Settings → Spam Filter → General → Quarantine Settings → “Delete Messages from Quarantine after X days”
By default, VPOP3 will create daily database backups called DBBACK-x.DMP in the VPOP3 folder. These are backups of the database.
In version 5 and later, all messages are stored in the database, so the DBBACK-x.DMP files may be large, as they contain all the messages (plus other data) In version 3 and 4, the backups will be smaller, because they only contain a partial message backup (so you should also backup the VPOP3 directory structure separately)
You can move the database backups to another location on the Settings → Database → Backup page in the settings (see also, the handling database backups topic), but we strongly recommend that you don't disable database backups totally. Otherwise if there is database corruption for any reasons, you will lose everything
VPOP3 also stores lots of other data, such as logging data, statistical data for the spam filter, etc. In versions 3 and later, this is all stored in the database, so will also be backed up daily.
If you have upgraded VPOP3 from previous versions, there may be files or directories which are no longer needed. The installer will not automatically delete all these files in case there are problems with the upgrade or in case people want to go back to an older version - most people don't make backups before performing upgrades!
Current releases of VPOP3 make a file called 'FilesToDelete.TXT' in the VPOP3 directory containing a list of filenames which can be deleted. You can either go through these manually, or use some other software to run through the list deleting files (e.g. here, we would use Take Command which lets you run 'del @filestodelete.txt' to delete all the files listed in that file).