Why does 'My Files' not explain why my SD card is full

I have an 8 Gb SD card showing 1 Gb free.

I know my SD card is storing some media, but the directories shown in the My Files app do not come close to accounting for the space used. I tried showing hidden files and deleting ".nomedia" files, to no avail.

What's a good app to figure out what is taking up all the space? My Files is not going to cut it. I want to zero in on my large directories.

File manager apps won't show hidden files or system files (unless the phone is rooted). Even apps that show where all the space is being taken won't.