is it normal on iPhone 15 pro that my backup is only 1GB when I use 50GB?

Hello Jerry2040,

We'd be happy to provide you with some information. Your device storage may differ from the size of an iCloud backup. "If you turn on an app or feature to use iCloud syncing (in Settings > [your name] > iCloud > Show All), its information is stored in iCloud. Because the information is automatically kept up to date on all your devices, it’s not included in your iCloud backup." You can learn more here: Back up iPhone - Apple Support

You can also see what's included in an iCloud backup here: What does iCloud back up? - Apple Support

Thanks.