Why is my MIUI Gallery using so much space and what to do about it?

Preface - In my Xiaomi Redmi Note 9 Pro with Android 12 I was shocked when I saw that my storage was almost completely full (128 GB). The more surprise that is was caused by Gallery (photo/video) app (com.miui.gallery). As I barely had 3800 photos and 70 short videos, I suspected _some app or something in the system_ was malfunctioning. *** Trial and error - I searched heavily for a solution yesterday. I tried so many things, it will be hard not to forget on many, just mentioning a few now: 1. Emptying all trash 2. Looking for hidden album(s) 3. Clearing cache Sorry, I don't remember any more. **The Gallery still took around 90 GB of space for no obvious reason!**