Well, how long is the movie and what is the movie resolution (ie: 640x480, 720x480, 720x576, 1280x720 etc)?
The longer the movie the more space it will take by default, and the higher the resolution, the larger the bitrate will need to be to make the movie look good on playback... You can always transcode your movie using MPeg Streamclip or Handbrake and attempt to reduce the quality/resolution/bitrate and see how small you can make it.