BTW, here's my ncdu screenshot:

  --- /mnt/src/6/linux-4.9-rc5 ---------------------
  
   401.7 MiB [##########] /drivers
   135.6 MiB [###       ] /arch
    37.3 MiB [          ] /fs
    35.3 MiB [          ] /include
    34.4 MiB [          ] /Documentation
    32.2 MiB [          ] /sound
    27.6 MiB [          ] /net
    13.4 MiB [          ] /tools
     7.5 MiB [          ] /kernel
     5.9 MiB [          ] /firmware
     3.6 MiB [          ] /lib
     3.4 MiB [          ] /scripts
     3.3 MiB [          ] /mm
     3.2 MiB [          ] /crypto
     2.3 MiB [          ] /security
     1.1 MiB [          ] /block
   880.0 KiB [          ] /samples
   424.0 KiB [          ] /virt
  [...]

I don't see subdirectories.

Linux source is here: https://www.kernel.org/