How to cCheck Size of a share/folder?

Something still does not add up (pun intended). If the explanation given reflects the mechanism correctly, it should also be impossible (or: not that easy) to show size of a single file. Yet under "browse" sizes of all single files are listed. Regardless of btrfs mechanism I fail to see why one can have, say, 20 files in a folder, each one showing, say 1GB size... yet supposedly there is no easy way to show the folder size being in total 20GB. Just calculate from the bottom up, add, rather straightforward I would say.

Yes, I am aware that interpratation of what the folder really is and how the data blocks are distributed is not easy, but for the purpose of showing how much space on the disk is consumed by a folder this is irrelevant. It becomes a bit mystical if we copy 100GB of data to NAS, to physicall disks, and from now on we are unable to say how much of this physical disk the data (and snapshots, if enabled) consume.