I have recently encoded a twenty minute video and forgot to resize the video (using the on2 encoder with flash 8). So the video encoded at 720 x 480, 30 fps and the resulting file was 90 mb. So I then changed all of the settings to resize the video to 360 x 240, 15 fps and the resulting file was only slightly smaller than the original 90 mb! Why is that? I figure the second file size should be about half of the first? What am I missing?
It may be because you didn't change your data rate. If your data rate is set to 400kb per second, it doesn't matter how big or what the frame rate of your video is, it is going to use 400kb for every second of your video, so if you cut the frame rate in half and reduce the image size, you will just get (theoretically) higher quality, smaller video.
The Flash Video Encoder lists it's data rate (Quality) as a Max. Data Rate, but I would guess that it would remain rather consistent with the data rate, given the same input video. If you got good results, but just too large of a file with your previous settings, and you reduce the image size and framerate, you should reduce the data rate (Quality) accordingly.