Used right-click Copy then right-click Paste to copy a large .iso file from an NTFS partition to an ext4 partition, using Thunar 1.6.3. The original file size was 828 MB. After being copied to the ext4 partition, Thunar showed it as 179 MB, a decrease of 79%. In the terminal it showed correctly. Choosing View | Reload did not work. When I renamed the file or closed and reopened Thunar, the file size changed to the correct value. I can replicate this bug over and over. Seems to happen with large files only.
Should be fixed in 1.6.5.