It's probably because the resolution of your clips are different. Check "properties" (in the map you've placed them) and check what resolution they have, for example 1020x1280. The movie you render should have these options set. Check
Fony's How to make a PvP intro for more information regarding this. If you still don't know what to do, just tell me and I will show in Sony Vegas. However, I think Fony describes it pretty good here.