iMovie doesn't appear to be able to use all the available space on iPad
I talked to several advisors at Apple Support, and eventually learned that this is a known bug that has impacted others besides me, and that engineers are working on a solution, but it is not known when that solution might be released.
Meanwhile the only workaround, since this is our only Apple device, is to download the clips that comprise this video from iCloud to another computer, edit the video again in an editing app that runs on that computer, and upload it from that computer.