How to stop Sony AVC/MVC encoder from adding 4:3 aspect ratio to 16:9 videos?
In project properties, set your pixel aspect ratio to 1.333 as well. Furthermore, make sure that when you right-click a video on your timeline that both resample is disabled, and try fiddling around with the "maintain aspect ratio" option. I have a hunch that it has got to do with these settings.

Try using different formats/encoders as well to see what effect that gives.