In Final Cut Pro X and Motion 5, the "Share to DVD" option automatically chooses the maximum quality bit rate that will allow your project to fit on the disc. The maximum allowable bit rate is 7.7 Mbps (high quality) and the minimum bit rate is 2 Mbps (low quality). The maximum duration that can fit on a DVD (per layer) is approximately 2.5 hours.
If you need more control over the quality of your DVD, use Compressor 4 to modify the automatically selected bit rate. For more information, see Compressor Help: MPEG-2 bit rates and formats and Compressor Help: MPEG-2 Encoder pane.
Another option when using Compressor is to choose two-pass encoding which will allow for better video quality since the bit rates are tailored to the video content. Note that sharing to DVD in Final Cut Pro X uses one-pass encoding.