I am a bit confused.
I encoded 28minutes of DV to DVD with Mpeg2enc at 5000kbit/s (with constant bitrate). The resulting m2v (video part)file size is 740Mb.

A rough calculation says it should be close to 1000Mb.

When I increase the bitrate to 6000 kbit, the reulting mpv file size is only 3% larger than the the one coded at 5000kbit.

Why is this, can someone explain?
Is constant bitrate not constant bitrate?

(I had the "Decode with Quicktime" and "Decode with mplayer" unchecked, FFmpeg should have been doing the DV decoding from DV PAL 720*576)

When trying with "Decode with Quicktime" enabled the noise level is higher and the image darker (close to black areas tend to clog). Why?

Also when using the same settings but decoding with Quicktime, the resulting file is roughly 30% larger.
Can this be right?
Why does decoding engine affect the filesize (mpeg2enc set to constant bitrate)?