Things happen—technologies fail, devices get damaged, or, god forbid, stolen. That's why it is very convenient to have a backup—you can restore all the data from it.
If you are attempting to restore a backup that is larger than the available memory of the device, unfortunately, you'll not be able to restore from the backup.
To fix the issue, we suggest you make sure you're using the latest version of iTunes and iOS. Updating your software may also resolve the ‘Not Enough Space to Restore iPhone Backup’ problem.