If you want to continue editing these files in iMovie, you'll have to deal with the huge file sizes, since editing can only be done with full-quality DV.
On the other hand, if you're done editing and want to store them or share them with friends, you can export the video as a QuickTime file. In iMovie 4, you can do this by choosing File > Share and selecting the QuickTime option. If you select "Expert Settings" you have a huge array of options that allow you to tweak file size vs. quality of the QuickTimes.