I have some curious behaviour of Avidemux.
If I setup a mkv Film to convert it into avi the wished Bitrate is not really token from the settings inside the Project imports.
There has been some similar thread from another user, but I could not find it.
I tested it and made savings from the MediaInfo from the source, the Project exportings from Avidemux and
the converted files.
The source is about 4.3G mkv, the target is avi (xvid) with first time Bitrate = 1000 and second time Bitrate = 2000.
But the targets are not so high in difference at size and at Bitrate.
The target with Bitrate=1000 gave about 1.2G/1000Kbps, the target with Bitrate=2000 gave about 1.4G/1317Kbps,
but the source should be high enough (2909Kbps).
I put the described Info into attach to view the related settings to get a better idea.
(Because I have been only allowed for four attachments I put the target results together in one file.
But you can compare it the same way).
Maybe you can declare this effect, because I by myself do not understand this.