Are you handquilting? I know that's what you do when handquilting- as Lisa said, it will spread as it gets quilted from the center out, and you don't want to be caught short. Also, it's possible that it could shift a bit to one side or the other.
I fold the excess over to the front and baste it down while in the quilting stage, it protects the edges and helps keep them from fraying.