Are you using DVD-R or DVD+R?
DVD-R is approximately 7MB larger than DVD+R. I know 7MB is not much but it's enough to have once given me a similar issue to yours.
7MB would not be enough to cover the extra size of the ISO. From what i have found, an ISO of course, is an exact image of the disk And also the layout of the disk which is where the extra size comes from. I will just have to use a DL.. i was on-site at the time of post so i was scrambling to find a solution due to no DL.
Thanks.