iCloud backup consists of more than just photos.
You can check what is using the space and how much it is using here:
Settings > iCloud > Storage > Manage Storage > select the backup for that device
Here’s what iCloud backs up:
- Purchase history for music, movies, TV shows, apps, and booksYour iCloud backup includes information about the content you have purchased, but not the purchased content itself. When you restore from an iCloud backup, your purchased content is automatically downloaded from the iTunes Store, App Store, or iBooks Store. Some types of content aren’t downloaded automatically in all countries, and previous purchases may be unavailable if they have been refunded or are no longer available in the store. For more information, see the Apple Support article iTunes in the Cloud availability by country. Some types of content aren’t available in all countries. For more information, see the Apple Support article What can I buy from the iTunes Store in my country?.
- Photos and videos on your iOS devices
- Device settings
- App data
- Home screen and app organization
- iMessage, text (SMS), and MMS messages
- Ringtones
- Visual Voicemail
Your iOS device backup only includes data and settings stored on your device.
It doesn’t include data already stored in iCloud, for example contacts, calendars, bookmarks, mail messages, notes, shared photo albums, My Photo Stream, and documents you save in iCloud using iOS apps and Mac apps.
It doesn’t include data already stored in iCloud, for example contacts, calendars, bookmarks, mail messages, notes, shared photo albums, iCloud Photo Library beta, My Photo Stream, and documents you save in iCloud using iOS apps and Mac apps.
iCloud: iCloud storage and backup overview