Moomin said:
No it won't, cutting the resolution will cut the file size. It's also true that lowering the bit rate will also cut the file size, doing both will reduce the file size a lot.
Bitrate means just what it says on the box: its the rate of the bits per second used to encode the data. Bitrate doesn't care what the resolution is, or what the framerate is. If you tell a video encoder to use 700kbps to encode a video, its going to do a better job on a file with less resolution, or slower frame rate, than on a file with a larger resolution for faster framerate, but it's going to use 700k per second to do the job, because that's what you asked for. (There is an upper limit to this, of course - an encoder will not use more bits than necessary to encode a frame.) So, simply changing the resolution without changing the bitrate will have very little effect on the final file size. You have to change both.
It should be pointed out that even though 320 x 240 is one quarter of the pixels of 640 x 480, you cannot reasonably only use one quarter of the bitrate to encode it. The reason for this is that each pixel in the smaller video is more likely to be "different" than it's neighbours in the larger resolution version - the details are closer together. Another way of thinking of it is that each pixel in the smaller video has more "work" to do than the pixels in the larger video. So, at best you might be able to reduce the bitrate to a third or half before you start seeing unwanted artifacts in the output video.
