It appears you have a mismatch of pixel aspect ratio [PAR] going on somewhere in your workflow. You could also have a PanCrop control in effect.
Check to make sure your Project settings match your Media being used. If you are rendering to a template, make sure you have the fill letterbox filled checked.
There are many places where you could be achieving this result.
I’m sure others will chip in here and offer help.
Grazie