> this could be seen as apple bloating figures

No, the OP doesn’t take into account that adding media to Photos is not a simple “copy and store a file”. The Photos app (like all apps) has its custom Photos Library file format. So when one ads a picture or a video, the Photos app analyses it and stores all kinds of metadata that’s needed for the Photos app to work, including edit history and other bits. This is what eventually gets synced to iCloud.

If you edit a photo/video in file rather than saving as a new file, iOS retains the original file so that you can undo/revert. It doesn’t show the original anywhere in Photos or iCloud gallery. That might be the reason.

Funny, this was may take on the article too. I suppose one could download x amount of files from one cloud storage and upload it to another to see if there is any obvious discrepancy. Perhaps Apple are still using their old file system format for icloud? It's been around for a good few years now and perhaps they simply haven't bothered to ever change the type of storage format they use. Maybe those with older accounts have their images / videos on the 'older' drives?