Bitrate of rendered video output is way higher than specified/possible

I recently rendered an animation, and the output file, which is only about 15 seconds long, ended up being 563 MB. This is a huge file size considering the resolution is 960x540 at 24 frames/second (the defaults for Blender 2.78a). The data rate (bitrate) of the output video file however was 298614kbps, or about 300Mbps. I’m no expert but I believe this is an astronomically high bitrate, more than 10X that of Blu-ray, and way beyond blender’s supposed encoding limits. I checked the project’s encoding settings, which specify a bitrate of 6000 (kbps) - again, the default value. Any idea what could be causing this issue?