Can iMovie Projects be stored in iCloud?
Choose File > Open Library > New.
  
  In the Save dialog that appears, type a name for the new library, and
  navigate to the location where you want to save it.
  
  The default location is the Movies folder in your home folder.
  
  Click Save.
  
  A new library is created in the location you chose, and a new, empty
  event with today’s date is created.


https://help.apple.com/imovie/mac/10.1/#/mov3fa25bae7

I don't have iMovie, so you need to check if iCloud is allowed there. If not, you need to copy paste items from ~/Movies to iCloud. 

As for iCloud backup that you mention, it could be system preferences etc that are backed up, not the Library itself.