Unfortunately this happens in window mode, yes, but we didn't expect that for fullscreen. The problem with windowed mode is that we have a 15:9 window, and Mplayer automatically stretch to fit this size. We need to figure out how to have mplayer to keep aspect ratio even if output window is different (both KMplayer and other said this is impossible) or detect video as 4:3 and create the window in the correct aspect. sorry about that :-(
switch_ratio <ratio>