If you type -b 10k (stupidly low) into the bitrate for a large movie, it will always be higher which sets a minimum bitrate for that image size. The codec needs that minimum bitrate to satisfactorily draw a picture with its most basic level of averaging.
ffmpeg -i fluid.mov -vpre libx264-hq -vcodec libx264 -b 10k 10kout.mov
= 150 kbps at 1024X576
Also, on the flipside, if you type -b 100000k, you will get a lower bitrate. (eg. 720x576 usually peaks at about 10-12 Mbps for mpeg2)
The codec has done every fancy thing it has up its sleeve and has nothing left to do so it needs no more bits.
ffmpeg -i fluid.mov -vpre libx264-hq -vcodec libx264 -b 100000k 100000kout.mov
= 9340 kbps at 1024X576
From the testing I have done with ffmpeg, it seems any video has a minimum and maximum bitrate based on numerous factors. (content type, movement, frame size, frame rate etc.)