The Halo 2 prerendered cutscenes are most likely 24fps.

As far as disc space goes, standard Blu Ray video is 20-30 Mbps. Let's say 25. That's 11 GB per hour of video (note this does not include the audio). If they use a lower bitrate with bink video or something, they can cut that down. If they did them at 5Mbps, that's 2.2GB per hour and a much smaller part of the disc.