Watching a widescreen DVD (non-anamorphic) will be native 16:9, so it should fill the screen when you press 16:9. If, however, when watching widescreen TV, when you press 16:9 you get a horizontal stretch that is not natural, the only thing I can think of is that the broadcast really is not native widescreen (i.e., 6:9) but rather is native 4:3 and probably not HD then. You may be getting 4:3 commercials in what is otherwise a 16:9 broadcast. It might stretch those commercials out of proportion.
Widescreen = 16:9. Stadandard = 4:3. Anamorphic or Cinemscope = 2.35:1 or 2.37:1 or 2.40:1, etc.