The file size shown in the window is the actual file size on disk for the cursored file.
A LMB click in this window will sort the active file window with an ascending sort on file size. The simulated LED immediately below it will be green.
A RMB click in this window will sort the active file window with an descending sort on file size. The simulated LED immediately below it will be red.
If the active file window is sorted on another parameter, the simulated LED below it will be gray.