"Keep aspect ratio" only ensures that the ratio is kept while resizing the playback window.
Aspect Ratio -> "Original" should return the physical resolution of your video without any AR adjustments made by BSPlayer.
Maybe your decoder is resampling the video to the correct aspect ratio but still carries the AR property along in the DirectShow filtergraph making BSPlayer do resizing when none is needed?
What is the physical ratio of your MPEG-2 video files (720*576, 720*480, any other)? What is the [aproximate] resolution your files
get displayed? What is the resolution you
want them displayed (720*404)?
That's lot of questions, but people, like the author of
this very recent post, manage to get files of very odd resolutions.