I am trying to render a video using MPEG2 CBR. I calculated the bitrate, and after a 3 hour render came to find the video file was too large. So I reduced the video, and rerendered. 3 hours later, the video was the same size. To do a few tests I rendered a smaller section at (SVCD)
2.62 Mbps
2.40 Mbps
2.10 Mbps
All of them are the exact same size!

I even saved a new custom profile after each change.