I've got a MP4 video file that's around 1.5GB.  It was shot in 1080HD.  I want to trim it down so I can burn it on a CD that will only handle around 700MBs.  I don't have a problem using the editing tools to get it to about half it's length, but when I go to save the project it ends up being just as large, or larger.  How can I just trim it down without increasing it's file size?