1) defrag defrag defrag
2) some of the changes are structural changes such as renaming fields or adding new fields needed by the emu engine, not just content.
3) I use
http://www.mysqlautobackup.com/ for my backups. I have backups run every 30 minutes and the software compresses it then copies it off site to another server i have at a separate data farm. 20 gigs of space is well enough for 30 days of running backups.