Can someone explain this to me
 

I downloaded a backup of my sites from a host in .tar.gz file format and when I decompressed it and check properties it shows Size: 396 MB and Size on Disk: 826 MB. Why would there be such a large discreptency in the size?

text files compress alot

Yep, text files compress a lot.

This has nothing to do with compression. What SexxxySites said is that when he DECOMPRESSED it, it showed file size : 396 MB, and on disk: 826MB. A decomressed file is a decompressed file.

Chances are, Sexxxysites, you did not grab all of your files. Better check again.

If you find all your files and are 100% you've compressed then decomressed them, you may still notice a file size descrepency, this may be because the term "megabyte" is ambiguous because it is commonly used to mean either 1000 bytes, squared, or 1024 bytes squared, depending who you ask, or what program is reading the disk, or even what operating system you are using.

And FYI, When used to describe data transfer speed as in MB/s (not Mb/s), it refers to one million bytes (not 1024).

Thus ends the lesson for today.

diffrent file system ?

Yes...after I decompressed the tar.gz file and used Windows XP Pro properties check it shows that the decompressed files to be Size: 396 MB and Size on disk: 826 MB.

I know that there is usually a difference in size when doing a property check but this is the largest difference I have even seen. In addition I have now discovered when using my virus scanner that a folder has a zipped file that shows a trojan. Is there any good reason that a host would have a trojan on its server.

I am putting the trojan into quarantine and do not intend to unzip it then I will email my host and question them about it.

Without getting into the trojan subject, generally a great ratio of small files (just a few kB ones) does that... "cluster waste" in the filesystem You decompressed to.

That is probably the correct answer as I have thousands upon thousands of small text files. BTW, the trojan was in my mail folder so I assume my host is not responsible for it.