windows backup

[#1]

Installed windows 7,Made a system restore disk and did a backup to an additional hard drive.
Out of curiosity I also decided to do a backup to a DVD.
There was the usual message to say mark this as disk one etc..the total space required is 12GB. I assumed this would span across 3 disks but it seemed to go on one disk which was also verified as OK.
Does windows 7 compress or alter the file in some way to reduce the size of the backup. ?
There was nothing else on the computer no other files other than windows 7.
Just curious ?

[#2]

Mines has done this and I only needed the one DVD, I think thats just a rough estimate on how much It could take.

[#3]

yes, that behaviour sounds normal.
If you do a full backup including the system state, the estimated (and actual) space required can be very pessimistic.  If you have installed any software (programs, drivers, etc) and a drive other than C:, you could find that it thinks it will need to backup those other drives too.
The backup tool is a volume-based tool, rather than a file-based one as was the case with earlier versions of Windows.  You'll need enough disk space for it to do a snapshot first, before it than copies the files to the DVD.

[#4]

That makes sense..... I did a back up to a second hard drive before the backup to DVD so I guess it might have figured I wanted to include the contents of the second hard drive as well.
One reason for asking the question was to reassure myself that the backup to DVD was ok. It verified the dvd with no error messages so your added explinations of the need for only one dvd was helpful
Thanks