Those are the thumbnails that are created based on your image count in order to load the pictures in album faster. I'd recommend to first move all your photos to some other storage (like PC) and THEN delete this file. Otherwise it's just like fighting with the wind. You delete it - it gets created again. All the time. Seems pretty pointless.
I the system create them after few time but sometimes the files size get more than usual. Sometimes it's get up to 2GB for every files and that's huge
If you don't want to do that try ES File Explorer
And you will see the folders and there sizes... And delete what is taking space
***NOTE*** ENABLE THE "SHOW HIDDEN FILES AND FOLDERS"
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