Have you tried re-starting your phone and/or doing a hard re-set and then checking storage again?
https://support.apple.com/en-us/HT201559
https://www.macworld.co.uk/how-to/iphone/how-restart-iphone-or-ipad-3662903/
If that doesn't help I would suggest deleting Notes (which you can now do in iOS 11) and then re-installing it from the App Store.
Delete built-in Apple apps on your iOS 11 device or Apple Watch - Apple Support
Make sure you search exactly for "Notes" and get Apple´s own app, which should be the first item found! Just for reference: I use Notes extensively on an iPhone X and have 511 notes, many with graphics and diagrams. Notes itself is only 13.6 MB, and all my data 30.4 MB. Hope this helps you! Regards, Ian.