Piping list of files and directories to du only shows sizes of directories?

What's going on is that xargs puts (if it can) all of the names on one command-line, so that you see only one command passed to du. Then du ignores the filenames (as you might expect: the files are part of the directories and it does not count those twice).

If you use a -n 1 parameter to xargs, it will split the command up and you will see something more as you expect.

find . | xargs -n 1 du -a