This message is being generated because Write Data does NO processing of the files (including compression, which would be necessary to make DVD9 fit a DVD5). When using Write Data, the files must be 100% ready to burn. You would need to use the ISO as a Source in Full Disc mode if you want to fit the files on a DVD5 (or you could write them in Full Disc to an ISO that would be compressed to DVD5 size, then burn that either direct after it compresses or with Write Data). Read the mode descriptions here. That might clear up some of the confusion.