Step 2: Know Thy Backups
Backup 3 can create two kinds of backup files: Full backups and Incremental backups. As you might guess, the first time you execute a Backup Plan, Backup automatically creates a Full backup; subsequent backups are of the Incremental variety and contain only files that have changed since the previous backup. For even better incrementality, Backup saves the two most recent versions of files that have changed. If you're still annoyed that Backup uses boatloads of disk space to work its magic, take heart: Incremental backups are way smaller, and thus require way less disk space than Full backups.
The first backup (left icon) is automatically a Full one; you can force a new Full backup any time. All subsequent auto-backups (right icon) are of the smaller, Incremental variety.
