Folder of 24 gigs has only 12 gigs nested. How to account for mysterious bloat?
tl;dr


Use du tool to see the truth.
Or, maybe wait a couple of hours.


Details

While shooting a screenshot of the Finder > Get Info showing 24 gigs and the results of the du command-line tool showing only 11 gigs, the Get Info panel changed with an automatic update. This was after a reboot and after 1-2 hours of repeatedly showing 24 gigs. 

So the Finder must be caching information about the folders long after deleting gigs of data in a large numbers of files.

As suggested by tubedogg, use the du command-line tool to see accurate readings of your folder sizes.