Linux – is size on disk so large for a single file

hard drivelinuxwindows 8.1

I've read a couple posts explaining that when you have many small files, the "size on disk" reported by windows can be much larger than the reported "size". This makes good sense to me, but from what I can tell, the "cluster size" or "allocation unit" is typically 4 kB, which (if I understand the argument right) means that a single file should not be more than 4 kB bigger in the "size on disk" metric.

I have a tiff image that is reportedly 65 kB in "size", but 1.00 MB when measured as "size on disk". What could the cause of this big discrepancy be?

Update: I realize now that the file is on a NAS drive that runs Linux. I checked the allocation unit size and it is just 4 kB, not 1 MB, for example:

bash-3.2# /sbin/blockdev --getbsz /dev/sda1

4096


I also checked a file that has "size" 1 kB and it shows up as 1.00 MB under "size on disk".