Welcome to the forum.
It could be either a setting on the TV (more likely), or on the DVD player (possible).
I would check on the TV's menu, to see what the options are, and which one has been chosen.
Now, to make sure that I am on the same page, as you, did you set up a Widescreen 16:9 Project, and author to a Widescreen 16:9 DVD? The authoring should be exactly what your Project was set to.
When you test the DVD on a computer, with a DVD software player, such as the free MediaPlayer Classic HC, does it display as Widescreen 16:9? If so, then it is very likely a setting on the TV, or the DVD player.