I would suspect it works in a similar fashion to this[1] modification of WinDirStat, which parses the MFT to do rapid scanning, but loses the ability to selectively refresh parts of the tree.
Wow, that's way faster than WinDirStat. Thanks for the link! This is awesome.
It's very fast, thanks for the recommendation. It's about page says it's so fast because it reads the NTFS' MFT directly. And it's written in Delphi.
It unfortunately doesn't seem to work properly with Storage Spaces. It can account for about 2% of the used space on my 50%-full thin-provisioned D: drive.
Not sure about Storage Spaces, but for non-local files in Onedrive on Windows 10 there are two size columns in WizTree for size and space on disk. Not sure it lets you select which value is used for ordering though.
It has the best UI by far of all the tools I've tried in this space. Unfortunately not open source.
For Windows my favorite tool is SpaceMonger version 1.4 (last freeware version). For some reason I find the layout algorithm very pleasing and the nesting easy to read. Other apps tend to put too many little rectangles too close together and it becomes unclear which files belong to which directory. Also I really dislike the popular "pillow" effect.