Sounds like the data DVD has benn optimized, meaning that duplicate files are actually pressent only once on the entire DVD. Whenever another instance of the duplicate file is needed, only a "pointer" is actually needed. For example, if you have several Windows Installer Setups on the DVD and each setup folder contains the "instmsiw.exe" (about 1.7MB), it actually is only exists once on the DVD...any other instances of the file point back to this file. In that way, disc space is reduced. Long story short, you can use later builds of Ultra Iso to potimize the folders and create the ISO and see the reduction size. FYI, in Ultra ISO you'll see that the ISO image would say 23GB or so on the properties, but the actula ISO Image file may be small enough to burn on your format of DVD. Hope this helps!