Avidemux not respecting target size option on Xvid?

[Aquous]

Hi

I have a bunch of mkv videos that I need converted to Xvid for space saving purposes. In Avidemux (using the Qt frontend on Gentoo Linux amd64) I open the mkv file, select Xvid for video, and choose 'video size (two pass)' in the encoding mode submenu, inputting a desired size of 285 MB. However, when I then start the encoding, the resulting video file is way larger than I specified - demuxing the audio and the video using ffmpeg reveals the video to be a whopping 323,4 MB in size, which is not what I specified!
What am I doing wrong?

[Agent_007]

What does AVIdemux log file say after the encoding is completed?

[Aquous]

I've attached the log. Thanks for helping me out

[Agent_007]

Unfortunately the log doesn't reveal anything helpful, since it seem Xvid or AVIdemux doesn't log certain selections.

[Aquous]

Anything I can do to change that? I have no problem with recompiling the thing from source, if that's what's needed to be done.