Mac: 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!