Why the size screen is different after I put them on DVD?

Hey Guys
I convert file avi to DVD with Roxio and Nero Vision Express and in the both case when I burn them on DVD the size of screen is different with what I have on my hard drive. Can someone can tell me why? Can I do something to correct that? Thank You for your answer?

A little more info would be useful.

What size are they on the PC - 4:3, 16:9, anamorhic etc? And how are they on the TV?

With GSpot give us the frame size eg 720x390.