I don't have a DVD drive, but my mplayer plays AVIs just fine in both fullscreen and 2x mode. Then again, I'm still using 3123, too...
Which input codec library does your version of mplayer use for AVI files? When I installed it (from CVS checkout from their servers, BTW), there was an option to get a better version of libavcodec, that (according to mplayer docs) supported the vast majority of its formats a good deal better than anything else -- but that libavcodec had a separate CVS repository somewhere else. So if the people that built your version of mplayer didn't use that libavcodec, it's possible that that could cause it. Maybe.
Of course, if you're seeing it in Ogle too, then maybe not. Does changing to 3123 help?