I\'m trying to figure out how to convert a file\'s (or directory\'s) byte size into kilobytes, megabytes, gigabytes, etc... respectively according the file\'s or directory\'s si
The file is 4 KB because that is the default block size of the HFS+ formatted drive. No file can be smaller than 4 KB. I suspect that is why your directory is a lot more than rounding up to 4 KB, since every smaller file in that directory is already "rounded up" to 4KB chunks, added together the difference may indeed be substantial.
As for your 1,315,291,136-byte showing up as 1.33GB -- is it really a monolithic file? Many files in the finder, for example applications, are actually a directory of many smaller files abstracted in the finder. If you open a contextual menu on the file (right click) do you see "Show Package Contents" as an option?