Excessive size of Mobile Documents folder

During the backup my hard disk using Carbon Copy Cloner, I was surprised at the time needed to back up the Mobile Documents folder in my Library folder, and the upload dialog's reference to apps that aren't even installed on my Mac - see below.

For some reason Finder would not show any contents within the Mobile Documents folder, even after running scripts to show hidden files.

A Get Info nonetheless indicated that the folder is over 18 GB!

Why is the folder 18 GB?? That's what I want to know!

Firstly (and this is where other community threads are inconclusive), my used iCloud storage only totals 16 GB.. of which 15 GB is purely iCloud backups of my iPhone on iPad. There is no reason or benefit of storing these backups on my Mac.

Secondly, after listing the folder's contents using Terminal, it seems that the folder is storing three other categories of documents locally:

i) iCloud Drive documents - THIS IS OKAY (though not sure why Spotlight files are also here)

ii) encrypted cache files for installed iOS apps - THIS IS NOT OKAY

iii) preference files for installed and uninstalled iOS apps - THIS IS ALSO NOT OKAY

I suspect that the cache files are responsible for the largest misuse of local hard drive space (just google for the issue that whatsapp has with com.apple.bird cache files..).

If anyone can enlighten us on the benefits or need to store these files locally, or confirm that this is a bug, it would be much appreciated!