Gbs back uo on Mac smaller on GB's than it shoud

Also, as long as your current iPhone is running at least iOS 12.4 (it can), you can transfer data directly from your old to your new phone, no computer involved:

https://support.apple.com/en-us/HT210216#migration

The iPhone backup does not include everything on your iPhone.

About backups for iPhone, iPad, and iPod touch

A backup from a computer doesn't include:

• Content from the iTunes and App Stores, or PDFs downloaded directly to Apple Books
• Content synced from Finder or 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.)

iOS and your Applications will not be backed up, because they can all be downloaded again from the original sources. (and also can NOT be restored from a backup on the iPhone, which is a closed system).

Compare to the graph that shows types of files used and relative sizes, not the total amount used. "Your files" are things like documents and photos.

EDIT: neuroanatomist's much more eloquent and explicit answer was not present when I started composing. Nothing I wrote was intended to contradict their statements.