I seem to remember being able to delete rendered files or other files that would help reduce the overall file size?
This wouldn't be for video that I'm currently working on but rather something I've already completed and put into the archives.
Another approach I use is export a master file from FCPX and use an external app to encode. I use Sorenson Squeeze. As mentioned, mp4 is the most compatible format. In Squeeze and other encoding apps you have much greater control over the encode parameters.