iCloud photos are not part of the backup of your iPhone to iCloud.
Read Apple's support document What does iCloud back up?
You will see:
Here’s what iCloud Backup includes
- App data
- Apple Watch backups
- Device settings
- Home screen and app organization
- iMessage, text (SMS), and MMS messages
- Photos and videos on your iPhone, iPad, and iPod touch
- Purchase history from Apple services, like your music, movies, TV shows, apps, and books3
- Ringtones
- Visual Voicemail password (requires the SIM card that was in use during backup)
Your iPhone, iPad, and iPod touch backups only include information and settings stored on your device. They do not include information already stored in iCloud such as Contacts, Calendars, Bookmarks, Notes, Reminders, Voice Memos4, Messages in iCloud, iCloud Photos, and shared photos. Some information is not included in an iCloud backup but can be added to iCloud and shared across multiple devices like Mail, Health data, call history, and files you store in iCloud Drive.
The key thing to note is footnote 2 which says: When you use Messages in iCloud or turn on iCloud Photos, your content is automatically stored in iCloud. That means that they're not included in your iCloud Backup.
Be aware that at some point you will probably need to increase your iCloud storage space to the 2TB plan.