DTV 480p is correct for the Xbox. The rest sounds like configuration issues on the display end.
Now for DTV 480p mode. In 480p passthrough mode, my TV doesn’t resize the image properly, so the picture appears zoomed in but at the correct proportions.
You need to toggle the aspect ratio on the display to a mode that disables overscan, like “1:1”, “native”, or something else.
In 480p line2x mode, if my TV thinks the picture is 1440×960 it shows me the entire picture but stretches it to widescreen.
You need to toggle the aspect ratio on the display to 4:3.
If your display doesn’t let you toggle aspect ratio (rare, but happens), you’ll need to either add a video processor that can control aspect ratio (perhaps by pillarboxing) or use a different display.