Diff video bitrate and MPEG-4 (base Media) value? Reencode and shrink old Flash vid?

[thoste]

When I inspect some *.mp4 video (with AVC H.264 and AAC codecs) with well known MediaInfo tool then there are two similar values (see attached snapshot):


One bitrate for MPEG-4 (Base Media):      In sample 19,7 MiB
One bitrate for (first) video stream):    In sample 5907 kbps

Whats the difference?

Sometimes I saw MPEG-4 (Base Media) bitrates with more that 200 MiB (!) which is blowing up a 3 minutes video up to 250 MB file size.

When I re-encode such a video in Avidemux (with H.264 and AAC): How can I reduce the MPEG-4 (Base Media) value?
Lowering the video bitrate does not help (much).

The original file (before first encoding is a *.m4v video file (=old Flash Video) if this helps

You cannot view this attachment.

[szlldm]

The 19.7 MiB is the size of the file (equals to 20.6 MB)
The 5907 kbps is the bitrate of the video stream