I now did the backup in simple mode, and the result is the same: i.e. the space on disk is considerably larger than the file sizes, by around 50%-100%.

No, sorry, I am in custom mode. I can re-do the backup in simple mode, but as I remember it was the same. I thought custom mode was simpler than simple mode?
(The backup was only done a couple days ago, so I don't think file deletion would have much effect on it, especially given that no large files have been deleted since the backup was created.)

Yes, in this case that's all I need. But I AM using simple mode, and while my data occupies 200 GB, the data on the disk occupies over 300 GB. Crazy.

Sure:
1. I believe it's file level backup because I see a folder icon next to the name of the backup.
2. I'm using Cloudberry Backup Desktop Edition which I believe is no longer supported, so I guess it's the legacy version?
3. As to the settings, I have it on manual backup to a local external drive, custom mode, fast NTFS scan, back up all files in selected folders (but not system and hidden files or on-demand files), after 90 days delete files that have been deleted locally.

Thanks that worked!
Now, it seems that the amount of space taken by the backup is MUCH larger than the actual size of the files backed up. In the screenshot you can see that the aggregate file size is 14.9 GB, yet the size on disk is a whopping 28.5 GB! Any idea why?

In the welcome page you get a summary of everything. Your backup buckets, backup plans, etc.

I'm not sure, how can I tell?