Dave is on the right track. Assuming you are using FCP X, your video files are probably inside it. Select one of your video files, open the inspector ( ⌘-4 ), select Info and down the bottom of the pane you'll find information about where the file is stored.
If your video files are not in the library of interest, you should consolidate the library files. That is under the File menu. The next step I would do is to delete the generated library files. This will delete render files, proxy and optimised media, keeping only your original files. All the deleted media can be re-created by FCP X if needed. This gets your library file as small as it can be. You can then back this up.
The advantage of backing up a library file, as opposed to just the original video files, is that the library file will now contain all the original video files (done by consolidating the library files earlier), plus any audio and images you've used, and the project file. Audio, images and project file usually only make up a very small part of the overall project storage size, but having them consolidated make it much easier to re-visit a project in the future, compared to having to rebuild a timeline.