Hello Dastejomay,
First, we'd recommend reviewing the differences between the iPhone storage and iCloud storage: What's the difference between device storage and iCloud storage? Which may clear some things up for us.
The biggest take away, is that the iPhone storage is not the same as the iCloud storage. You cannot increase your iPhone storage capacity. While you can increase iCloud storage: Upgrade to iCloud+
What syncs with iCloud is stored in iCloud, not on the device.
Use these two pages to manage your iPhone storage and iCloud storage:
With that said, you should be able to see what's being stored in iCloud Drive using the iCloud website: iCloud Drive Sign in and look in iCloud Drive to see where these files may be located.
In case you continue to have issues, are you using Desktop and Documents with iCloud Drive? This would sync your Desktop and Documents from your computer with your associated devices, which may be why so much space is being taken by iCloud Drive.
Hoping to hear how it goes, take care.