The filesize is big because there isnt enough time to encode the movie while the game is running. The movie frames are slightly compressed, but still require a large amount of disk space due to the high resolutions. After you have finished capturing you can convert the clips into a compressed format using a video editing program. Probably the easiest one to use is Windows Movie Maker which is included free as part of Windows Live Essentials. It allows you to easily edit your clips and add text and other effects to your film. The end result will also be compressed into the Windows Media Video (WMV) format which can be played back in most media players.