Finder shows the wrong file size compared to Terminal or e.g. Gmail (not the base-10 vs base-2 problem)
Actually, the answer is in your "Get Info" window:



You've got compression enabled on that drive. The file does have 6MB data in it, and when you copy it elsewhere, that's what it takes up.

However, with compression enabled on that drive, the file only takes up 2.9MB on the disk because it has been compressed.