Thanks for your comments.
After opening up the contents of the library on my iMac I'm beginning to see why the Mac library is so much bigger than the iCloud library.
The number of pictures and videos is the same in both libraries, but the Mac library contains many GBs of Adobe Photoshop Elements files, which individually range in size up to 36 MB. In addition, there are many "Canon Camera Raw Files", and the files sizes are up to 16 MB. I often use PSE to edit pictures from within the Photos app, and in the past remember importing Canon raw photos into Photos. I'd have guessed that the PSE activities wouldn't be placed in the Photos Library.
I'm guessing that these files, although they are in the "Photos Library iCloud.photoslibrary" on my Mac have not been uploaded to iCloud. I'm not sure what good they are buried inside the Photos Library.