By default, the Finder's list view only shows you the size of individual files—not of folders. This small, somewhat hidden setting will calculate the size of entire folders so you can see where all your data is.
Reader Etudes explains:
If you get annoyed as I do that sometimes you can't see the complete size of a folder in Finder, here is the way to do it: Switch to list view (View > as List); right click on an empty space and open up Show View Options; check "Calculate all sizes." And voila, it will now detect the complete size.
Simple, but useful!
Detect Combined Size of a Folder (Including Subfolders) in Finder | Hackerspace