Mac storage basics · iPhone and iPad backups and updates
Why is my iPhone backup on my Mac so big?
Short answer: It includes app data, messages and attachments and, if iCloud Photos is off, your photos and videos. With iCloud Photos on, photos aren't included.
A device backup includes app data, messages and attachments, settings and — if iCloud Photos is off — your photos and videos stored on the phone. Apps that keep lots of offline content (messaging apps, podcast or video apps) make it bigger.
With iCloud Photos on, photos aren't included because they're already in iCloud. To make backups smaller, clear large app data on the phone or rely on iCloud backups and keep only an occasional local one.