The reason why it doesn't use the full sensor and scale down the video is cause it would require more processing power and would be of lower quality. That's a whole extra step it would have to do for each frame of video which means it needs amore powerfulCPU.Also, the quality would be slightly *lower*. If you think about it, if the highest quality video is only using 1920x1080 pixels of the sensor then it is mapping 1 to 1 pixel. Itshould result in a better picture than taking a higher amount of pixels and discarding a bunch of them to create a lower resolution. Doing that would meanyoulose1 to 1 pixel informationand insteadyou're leftwith aninterpolated image (downward interpolation). That would be of a lower quality.
I can't follow your conclusion. More information leads to better image! Certainly the CPU has to have the feature for down scaling (it has for other resolutions). The quality of a video 640x480 is better then a detail 640x480 out of bigger Video (for example 1280x720)!