It would be helpful if you could post a screen shot of what you are seeing.
Your issue may be the same one that a couple of other posters have described recently. See the threads below:
iMovie not filling screen - Apple Community
16/9 on imovie 10.3.3 - Apple Community
I had recommended that the posters in those threads try deleting preferences, but since they never posted back I don't know if the procedure cured the issue. I suggest that you try deleting preferences and see if it cures your situation. To delete preferences, close iMovie then reopen iMovie while holding down the option and command keys, and selecting to delete preferences in the box that appears. iMovie will open in a new library. Re-open your old library to get back to your projects. This is a safe procedure and will not cause loss of data. If it doesn't seem to have worked, try rebooting and then check again.
-- Rich