I have deselected everything from iCloud backup except Contacts multiple times, deleting my backup in between. The instructions from Apple lead right away to buying more storage,
[Edited by Moderator]
Your iCloud storage holds a number of different things: iPhone/iPad backups, data you've synced including messages, contacts, photos. It also holds files you've put in iCloud Drive.
You can get some idea about what is taking up the space:
Check your iCloud storage on any device - Apple Support
You cannot exclude anything from backup except by sync'ing them to iCloud.
Backup will include everything not sync'd to iCloud.
Why do I keep getting iCloud storage notifications when only backing up Contacts?
