Select multiple files or folders. Open Disk Usage (Ctrl+Y). It only counts the last clicked file or folder. It should count every selected file or folder.
Several functions are not yet available for multiple selection of files, disk usage is one. Please add as a bugnote any others that do not work so that they will be included in forthcoming 4.2 version.
diskusage is now available for multiple selections in 4_0 and HEAD.