Video compression is a black art. There are many, many, did I say many, factors you can adjust.
File->Export...
To Quicktime.
Expert Settings
Export
Options...
Applying normal file compression usually doesn't do much. Video compressors know they are compressing video and understand that much information remains the same between frames.
If you selected DV output then there is no compression going on. DV is frame for frame the same quality that is on your video camera. Its about 15MB for 4 seconds.
The email, web, and CD options usually are good about getting the size right for what you want. To preserve the high quality, exporting to Camera tape is usually the best. I don't have a superdrive so I have no experience with the iDVD quality. I usually end up with about three exports per movie, depending on the varying audiences.
Good luck.