miDVD has to convert the video to the MPEG-2 format which is part of the original DVD standard, but is less compressed the more modern video formats available today. As a result of this the file sizes may expand, you may not be able to fit as much on the disk as you would expect.
Also miDVD has to estimate the final size of the DVD before burning. If you are very close to the limit, it may get this wrong and the burn will fail because the converted video is too large.