windows 10 - Strange Disk/Folder size issues (different results)

On windows 10 x64 machine, with WD Purple Drive 4TB

I have a MAIN folder located at

R:/Backup/Main Folder

inside the that are lots of sub folders such as resources, projects, and what nots.

Now, if I right click > properties on

R:/Backup/Main Folder

I get the following results: 9.46GB Size

But if i do the same for one of its subfolders

R:/Backup/Main Folder/Work Stuffs/Projects/BigProject/

I get this: 153GB!!!

Please note , no hidden files or anything under these folders, just plain files from images to videos, and scripts, etc.

How the heck can the subfolder have bigger file size and disk consumption than its own parent folders?? How can I get the accurate and real SIZE of this MAIN FOLDER then?