Mac storage basics · iPhone and iPad backups and updates
Where are iPhone and iPad backups stored on my Mac?
Short answer: In your Library under Application Support → MobileSync → Backup. Manage them in Finder: select the device, then General → Manage Backups.
Finder (and older iTunes) saves device backups in your Library under Application Support → MobileSync → Backup, one folder per backup. Each can be as big as the device's data, so several old backups add up fast.
The easiest way to see and manage them is Finder itself: connect the device, select it in the sidebar, and on the General tab click Manage Backups…. Storage settings' iOS Files lists them too.