backup size

There are several differences:

• The backup does not include apps or music, which can be resynced from a computer or downloaded from the app store
• The backup does not include any data that is synced to iCloud (check by going to Settings/[your name]/iCloud; any switches that are on indicate that the item is syncing, and will not be in the backup)
• Backups do not include data that is synced to external servers, such as email and contacts.
• The backup is a SQLite database, and has the overhead of the database metadata

Here’s more information→Backup methods for iPhone, iPad, and iPod touch - Apple Support

Because not everything stored on your phone is part of the backup.

Apps and media are not backed up as they can be re-downloaded / synced. It would be a waste of time and space to included them in the backup.