There are several different DVD types, and some can hold more data than others. You probably created an image of a dual layer commercial DVD. Images made from dual-layer discs cannot be burned to single-layer DVD-ROM, DVD-R, or DVD+R discs. See the “Types of Discs” chapter in the online Help:
http://www.deskshare.com/help/dap/TypesOfDiscs.aspx. You can purchase dual-layer DVD+R and DVD±RW discs, and you should be able to write your image to one of these.
DeskShare - Support