back up

Watch the iTunes BackUp Size on a GIF below

Note this is a compressed backup size. (During restoration it will expand)

See the GIF which shows as 28.03 GB used space on iPhone 6 and the Backup Size id 5.26GB

Same is true for iCloud Backup too. What you see the size is compressed Size

iCloud backup size on iCloud is 3.5 GB for the same iPhone same date that is because iCloud backup doesn’t include information that are synced and stored in iCloud, like Contacts, Calendars, Bookmarks, Mail, Notes, Voice Memos, shared photos, iCloud Photos*, Health data, call history, and files you store in iCloud Drive.

*-If you had turned on iCloud Photos, your content is automatically stored in iCloud. 

Watch the GIF below

What does iCloud back up? - Apple Support

I think I am unable to communicate.

Let my try again, irrespective of where your backup is (iTunes or iCloud) it will only show a compressed file size and you will not be able to estimate whether it will fit your iPhone storage capacity.

So knowing backup size isn't going to help you in anyway.

The backups already on iTunes are probably from an iPhone which had a larger capacity than the one you are trying to restore to.

Watch the GIF below

Please refer to my second post. on iTunes Backup.