Folder of 24 gigs has only 12 gigs nested. How to account for mysterious bloat? I have a ~/Library/Developer used by Xcode.app in El Capitan. The Finder > Get Info reports a size of 24 gigs. But the two nested folders each report only 6 gigs each. So ( 2 * 6 ) = 12, and ( 24 - 12 ) = 12. Where is the other 12 mysterious gigs? 

I am showing invisible files/folders. So I believe I have not overlooked any other content. I did recently delete much content from there, and emptied the Trash.

How can I track down this huge discrepancy?

Using the du, the “display disk usage statistics” tool, where -d is depth (Display an entry for all files and directories depth directories deep) and -h is "Human-readable" output (Use unit suffixes: Byte, Kilobyte, Megabyte, Gigabyte, Terabyte and Petabyte).

Outer folder:

cd ~/Library; du -d 1 -h;


…
 11G    ./Developer
…


Inner folders:

cd ~/Library/Developer; du -d 1 -h;


706M    ./CoreSimulator
4.4G    ./Shared
5.9G    ./Xcode
 11G    .


cd ~/Library; du -d 1 -h;

But the Get Info reports twice that, 24.x gigs.
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.