A nice free light app to monitor your disk usage by folder is this WinDirStat:
https://windirstat.net/
 
On start up, it reads the whole directory tree once and then presents it in three useful views:

• The directory list, which resembles the tree view of the Windows Explorer but is sorted by file/subtree size,
• The treemap, which shows the whole contents of the directory tree straight away,
• The extension list, which serves as a legend and shows statistics about the file types.

 
I really gotta wonder why Microsoft doesn't include something like this in the basic Windows accessories...