iTunes backup yields too low of an output file.
Why would my iPhone 6's 50 GB of internal storage data use up only 20 GB of desktop data after an iTunes backup?
Earlier iPhone models
Question marked as
The backup does not necessarily include every single thing on your iPhone. For example the backup includes email settings but not the emails themselves (the emails are generally stored with your provider). For more info, see the 'Backups from your computer' section of:
Backup methods for iPhone, iPad, and iPod touch - Apple Support
to see what is not included in the backup: