PiP box distortion

Hello,

If I reduce the size of several clips to 30% or so and place them on a black background simultaneously, the sides of the clips appear slightly warped or distorted on playback. It's not so noticeable with just one "picture in picture" effect, but 3 or 4 is pretty bad. (Playback is to a NTSC ref monitor.)

I've seen this on some broadcast commercials so I know it's not just my setup, but what causes that? Can anything be done?

Thanks,

Paul