[Alexey Shulev]

I found the roots of this problem, iMovie duplicates it’s library on the startup disk Library folder somewhere deep, so when you add some file to the library it takes up size twice. As you may know Imovie and Finalcut are pretty much the same in the way thew work with files, so check your library, you can use this app Disk Expert, it will show you all of this fast. It has trial version so it will be enough for you to check.
I hope it will help.