Hi,
The fact that the size of epesi directory increases with time is normal. Data folder holds all large objects like attachments (files) and if you use integrated Roundcube e-mail client and use archive e-mail feature it will also save file attachments. Every company is different and it is hard to estimate type and size of file attachments. There are temp files created like the cache of the minify folder, smarty template compiled files and so on. But they don't grow in size indefinitely once everything is cached.
The space is cleared during the update process so there is really nothing there you need to do.
Data folder and the database is where your entire data resides. You can drop it into new EPESI installation and have a running system. Therefore please make sure you have proper disaster recovery mechanism in place - a backup of at least /data or preferably the entire /epesi directory and a daily dump of the database should be storred off-site or at another location like for example FTP server or NAS drive, whatever you decide.
