My iCloud backup is smaller than actual GB being used in my phone

See the following from What does iCloud back up? - Apple Support

Your iPhone, iPad, and iPod touch backup only include information and settings stored on your device. It doesn’t include information already stored in iCloud, like Contacts, Calendars, Bookmarks, Mail, Notes, Voice Memos³, shared photos, iCloud Photos, Health data, and files you store in iCloud Drive.

1. When you use Messages in iCloud or turn on iCloud Photos, your content is automatically stored in iCloud. That means they're not included in your iCloud Backup.
2. Your iCloud Backup includes information about the content you buy, but not the content itself. When you restore from an iCloud backup, your purchased content is automatically redownloaded from the iTunes Store, App Store, or Books Store. Some types of content aren’t downloaded automatically in all countries or regions. Previous purchases might be unavailable if they've been refunded or aren't available in the store. Find out what you can redownload from iTunes in your country or region and what you can buy from the iTunes Store in your country or region.
3. If you use iOS 11 or earlier, Voice Memos are included in iCloud Backup.