Are you saying that a certain DVD looks better on your TV than your MBP?
If so, it could be because you're sitting closer to your MBP, because your TV has better output, or the DVD player does a better job upscaling, or all of these.
-Or-
Are you saying that you watch a movie on DVD on your TV, then watch it as an SD movie file on the MBP?
If it's this, it's because the movie file is most likely compressed down.