Why is my iTunes backup only half of my current iPhone storage?

yntheng,

Thanks for the additional Information. The article provided previously, About backups for iOS devices mentions some data is not included in an iTunes backup, as listed below:  

"An iTunes backup doesn't include:

• Content from the iTunes and App Stores, or PDFs downloaded directly to Apple Books
• Content synced from iTunes, like imported MP3s or CDs, videos, books, and photos
• Data already stored in iCloud, like iCloud Photos, iMessages, and text (SMS) and multimedia (MMS) messages
• Face ID or Touch ID settings
• Apple Pay information and settings
• Apple Mail data
• Activity, Health, and Keychain data (To back up this content, you'll need to use Encrypted Backup in iTunes.)"

If you have any of this data on your device, this may explain the difference in the backup file size.

If this still doesn't answer your question about the file size difference, I recommend you reach out to Apple Support:

Contact Apple Support

Best Regards.

Hello yntheng,

Thank you for using Apple Support Communities! I understand from the information you provided that you would like to know why there may be a difference in your backup size; I'd like to help.

Here's a resource that could help:

About backups for iOS devices

Specifically, check out the section labeled "iTunes backups"

Because an iTunes backup doesn't backup the content already in iCloud, I would recommend that you have a recent iCloud backup as well.

Hope this helps.

Best Regards.