Library folder listed size is 90 GB. Why so large
It depends on your settings. If you have it configured to store everything locally, you won't save any space on your internal drive by moving files to iCloud Drive. If you have it set to store them only locally as needed, then you can save storage space that way.

You can change the settings in System Preferences > iCloud and then "Options..." for iCloud Drive. At the bottom you'll see "Optimise Mac Storage" that you can check to request that files can be stored in the cloud only.