Lightroom Library.library is large and I need to reduce its size

Johan has already explained how to deal with the stored images that are stored in the Lightroom Library.lrlibrary, but in order to prevent this continually happening every time you import new images it would be sensible (if possible) to change the location for locally stored originals to an alternative drive (using the option on that Local Storage preferences tab).

The Lightroom Library.lrlibrary itself cannot be relocated, but the original images can be. Doing that would be a significant help to managing the space on your system drive.

Lightroom copies the originals into its library before it uploads them to the cloud. After successful upload, these local copies should be deleted, but that takes time, sometimes many months before it happens. I don't know why this has not been dealt with.

You can manually delete the originals if you are sure they have been uploaded already. Right-click on the Library and choose 'Show Package Contents'. That opens the library as a folder. Inside it there should be a subfolder with the originals.