The statistics Hash/File Information do not update in the Known Files section if you do Shift-Click or Ctrl-Click. So if you select multiple files or directories, the Size should show the total for all of the items selected.
You can even have only one item selected and have the statistics be wrong by selecting a file. Use Ctrl-Click to select another file and then use Ctrl-Click to deselect the original file. The information shown is not the information for the file that is selected.