If you are create Standard (4:3) DVD you may have to change your DVD or/and TV setting from an automatic mode to the correct aspect ratio - a lot of equipment nowadays will assume content is widescreen or try to "fit" the input to fill the screen. If you are playing an MP4 file you may need to create an upscaled file (which you can do in Studio), depending on what the TV is expecting.

There are too many variable here to know what is happening in your case, I'm afraid. DVDs can inform the player of their intended aspect ratio, simple file playback doesn't have this information. Does your file play OK in Media player on the computer? Do you have DVD playback software on your computer? Are you creating a DVD in Widescreen or Standard aspect ratio? Does it appear to have black masking at the sides or top?

If I were making this project I would use a widescreen project setting - NTSC Widescreen in your case - and place the source material on the timeline using the Fit setting so that it was the correct shape. Even then some video sources may need adjusting using properties so that the 4:3 material has back bars at the sides. Then I'd make a widescreen DVD and expect it to play correctly on a widescreen DVD/TV combination. I'd export a Widescreen mp4 file, or even a 720p file.