You may notice that your backup files created using iMazing are substantially smaller than the amount of disk space in use on your Apple mobile device. There are 3 main reasons for this:
1) Not all data is contained within your backup files. Media (Music and Videos) for example, needs to be Exported and Imported separately. We are looking into implementing a "Total" backup feature in a future version, but for now you need to select one of the media tabs in the left of iMazing and use the Import/Export options in the bottom toolbar. Note that iTunes backups similarly do not contain Music and Video libraries - the reason is quite simple: allowing duplicates of these libraries would consume a LOT of disk space.
2) To avoid duplicates, apps are stored separately from app data. The IPA files are stored within the Apps directory of your application backup, and app data is stored within the specific backup. Furthermore, on iOS 9 and above, only app data is backed up, not apps themselves.
3) Backups are compressed in various ways. For example, if you chose to include Apps in your backup, iMazing uses Apple's built in archiving firmware to package the Apps themselves (avoiding duplicates), then zips application data separately.
If you are unsure that all of your device data (excluding your music/video libraries) has been properly backed up, you can use our backup browser to verify the contents of the backup. Apps won't appear in a separate section, they'll be in File System->Apps. If still in doubt, delete the backup from within iMazing and backup afresh.