Thunar is displaying the incorrect file size for a single selected file, or a group of multiple files selected. Thunar is reporting the MiB of the file as the number of KiB/1000, which is the method that hard drive manufacturers use to dupe us into believing we're getting a larger hard drive than we actually get.

The MiB file sizes should be based on the KiB/1024 measurement, which is how the OS sees a MiB. By using a simple hand calculator, I can take the total number of bytes listed in the file properties dialog and divide by 1024. The result is the same number of MiB that are reported by other file managers, such as Dolphin under KDE, and by Firefox when I download that same file. The larger the file, the larger the discrepancy between what is reported by Thunar as the file size and how every other application sees the file size.