Another ffmpeg related finding on the web:

robert.swain » bit rate, file size, quality misunderstandings.

When working with ffmpeg I always find this article under the first in Google’s PageRank, and  it is really a good start if you have no idea how to understand video encoding’s inner workings.

The article ends in the explanation of -crf (constant rate factor) vs. -sameq (same quantiser (!) not quality).