I think there is something that folks aren't saying when they say that rendering a noise-reduced video results in a smaller file, and i think the thing they're not saying is that with noise reduction one can use a lower bitrate to obtain the same subjective quality that can only be obtained with a higher bitrate on a noisier video. And, since a lower bitrate can be used, the resulting file is therefore smaller, not because of the noise reduction, but because the noise reduction allows a lower bitrate to produce acceptable results.

It's the same sort of thing with folks wondering why when they switch from 640x480 to 320x240 the WMV file is still the same size. Of course it's the same size ... if they use the same bitrate. However, using a smaller frame size would allow a lower bitrate to produce the same subjective quality. So, reducing the frame size in conjuction with a lower bitrate will result in a smaller file. For that matter, using the same frame size with a lower bitrate will also result in exactly the same smaller file size.

Think about what "bitrate" means. It's the number of bits used each second. So, no matter what's in the video or what the frame size or how much noise or motion, using X bits per second for Y seconds results in XY/8 bytes. That's the mathematical way of saying that the resulting file size, under ideal conditions, depends on the length and bitrate only.