You’re only seeing part of the image because you have the preview set to 100% and your frame size is far bigger than the preview window, and therein lies your problem.
Why are you editing 720×480 in a 1920×1080 project? Naturally you are going to get a black border because the former is much smaller than the latter.
You can scale the 720×480 clip to fill the 1920×1080 frame, and then when you export to 720×480 it will still fill the frame, however all that scaling is so unnecessary.
Cheers
Eddie