Windows 7 backup

I am trying to backup to an 8G flash drive. Backup uses approx 7.3G. The first time I backup everything is fine. When I try to backup again it says I don't have enough room on the drive. Isn't it only supposed to backup files that were changed since the last backup? Or is this because I am trying to backup manually instead of letting backup start on it's own? Do I have too little space left?
Sometimes when I delete the existing backup the next backup is only 142mb. What am I doing wrong?
Stan

Hello!
I'm pretty sure Windows 7 needs so much free space on any place its copied to, so you would probably be able to get more free space on the jump drive if you remove some items within Windows that are not needed and can be reinstalled after a system restore has completed.
I may be wrong as you are only making a copy for back up, you may simply need to compress the files. Here is a link to making a system back up for windows 7....From this forum that I found for you!
Backup Complete Computer - Create an Image Backup

Hi,
I looked over the link you sent me and even though it wasn't exactly what I was looking for it did help me.
Thanx for taking the time.