There's two issues here:
1 - iDVD can only create SD (standard definition) video disks. What you can see in iMovie on your Mac's hi res monitor is way beyond the SD resolution of video DVDs.
2 - instead of using the Share ? iDVD menu option use the Share ? Media Browser option and then drag the movie from the Media Browser in iDVD into the iDVD menu being careful to avoid any drop zones. This will give you a better quality movie than the Share ? iDVD method.
Also follow this workflow in iDVD to help assure getting the best quality final product possible:
"Once you have the project as you want it save it as a disk image via the File ? Save as Disk Image menu option. This will separate the encoding process from the burn process.
To check the encoding mount the disk image and launch DVD Player and play it. If it plays OK with DVD Player the encoding was good.
Then burn to disk with Disk Utility or Toast at the slowest speed available (2x-4x) to assure the best burn quality. Always use top quality media: Verbatim, Maxell or Taiyo Yuden DVD-R are the most recommended in the Apple iDVD forum."