Windhawk made the size column in File Explorer finally useful.

Windhawk lets you add features, tweak UI behavior, and restore classic elements in Windows, and these nine mods make it shine, from per-monitor Alt+Tab and better File Explorer sizes to taskbar ...

The thing is, you don't really WANT Explorer to do that, because that means it's calculating the size of folders (and all subfolders!) all the time. It will slow to a crawl. That said, I'm pretty sure ...

Now, there would be no real speed penalty in detecting size if the file system or even Windows kept a record of the size of every directory (counting subdirectories), but there'd be a penalty for all ...