Open iMovie '09 .rcproject in iMovie 10
In iMovie 09 an RCProject file is a binary text file which contains instructions and references to the actual video material. The actual video clips are stored in an Event folder. Check to see if you still have your Event folders. The clips would be in there.

You mention that your RCProject file is large. This probably means that you rendered your finished movies. iMovie saved the "Media Browser" copy in the rcproject file so it doesn't have to render it again unless you make a change.

So open your RCProject file and navigate to the Movies folder. Any completed movies would be in there.

To open the RCProject, right click on it in the Finder and select SHOW PACKAGE CONTENTS.

One more thing: If you do find the Event folder to go with the RCProject folder, iMovie is finicky about where these files are located.

Put the RCProject file in the Movies/iMovie Projects/ folder.
Put the Event in the Movies/iMovie Events/ folder.

With these in place, there is a chance that iMovie 10 will see them. However, I am not sure if you have to convert to iMovie 11 (which is version 9) first. Both iMovie 9 and iMovie 11 use the RCProject format. The current iMovie is called iMovie (no year) and is Version 10. It uses a scheme more like Final Cut Pro, so everything is in an Event folder, including any projects relating to that event.