dumb question(s), but....
1- in your motion menu, is the video clip set to fill the *entire* background screen (720x480, or 720x576)? if so, it's not really a grand idea to do this (unless the original video is excellent quality to begin with).
in my own experience, after having authored a few hundred (yes, hundreds) DVDs, on the rare occasion when i do a menu with motion, i have the video clip fit on a small portion of the menu background. that way, even if it's not a great quality video (but in my situation it is, otherwise i wil not spend time on ultra-fancy menus), it still looks okay because the video clip only takes up one-fourth or less of the background screen.
2- from your post, it appears you're having TMPG DVD Author "render"/encode the motion menu... again, if you can use a different encoder, i'd definitely suggest it. on the DVD i just made, i used a 5 minute clip (full song), and encoded it at 4 MB/s (which is low for a DVD video bitrate), the size is right at 173 MB for that menu, and the quality is fantastic... of course, the original source was an excellent digital camcorder (masters > firewire > DV .AVI), and i used 10-pass encoding in CCE.
i don't know if TMPG DVD Author will let you simply import an already-encoded MPEG-2 video for the motion background (and not re-encode it), but if it will let you, definitely try it out. i'm sure you can get much better quality.
a similar thing had happened to me years ago when i was using Ulead DVD Workshop, and i had let it use its MPEG encoder to encode the background video. big mistake. MUCH better quality when i figured out how to import an already-encoded clip.
if you have Premiere or Vegas (or Final Cut, etc.), you can actually create the entire background for menus - complete with song titles, and then encode it to MPEG-2 (with nice quality). then with a decent authoring software, you can simply import that MPEG-2 video as the menu and add buttons or a subpicture (to highlight the song title text, for example).