Hard to tell without more information, but the simple answer is that you can't put 9 pounds of potatoes in a 5-pound bag. Are you trying to copy a commercial movie DVD? Many of them are dual-layer, and even if you get past the copy protection they won't fit on a standard DVD-5 (single layer disc). In that case, go down to step 3) below. OTOH if you're making a homebrew disk and your videos are still too big to burn, you can either

1) Eliminate one or more videos from the collection you're transcoding (if you're making a DVD with several videos/titles in it);

2) Transcode at a lower bitrate, which will squeeze more content onto a DVD but at the cost of lower video quality [click on "More" at bottom of the NeroVision "Content" screen, then Video Options, DVD-Video, Quality setting = Automatic (fit to disc) or try Super Long Play]; or

3) Burn to a dual-layer disc (DVD-9), with about double the capacity, if your burner can handle DLs [ask for DVD-9 in the drop-down menu, lower right corner of the NeroVision "Content" screen]. Warning--DVD-9s are trickier to burn and are more likely to end up as coasters. They also cost a dollar and a half each and up. I've found Verbatim DLs very reliable, and I've also had good luck with Memorex DLs, which many people despise, but which my Samsung SH-S222L burner handles with no problems. Circuit City sells 25 Memorex DLs for $30, and sometimes for $20. Verbatims go for maybe twice that.

4) Finally, you can try using DVDShrink to analyze and shrink down a commercial video to an .iso that fits onto a single-layer (DVD-5) disc, or, if you already have those too-big Nero .nrgs, use UltraISO to convert your dual-layer Nero .nrg file to a standard .iso file, and then use DVDShrink to convert that to a DVD-5 .iso file, which should fit on any blank DVD. Use Google to locate UltraISO and DVDShrink; the latter is especially tricky to find.

Good luck!