Welcome to the family, we were al new to this at some point.
Your .motn project files are like Word files, all they do is tell the application what to do. they have no affect at all on anything anywhere if they're not actually running. The resources (movies, stills, etc) attached to those project may be clogging up your external storage but they have no impact on your system at all. None.
"Od projects" have a horrible way of becoming current projects so be sure you're offloading them (and all associated resources) to recoverable storage.