What size batting and backing for a Quilt
I'm on my first quilt. I've finished with the front piecing. (It's basically a t-shirt quilt with those game towels they give out at games). I've been watching videos and reading places and they all say to have 4+ inches of batting and backing fabric on each side and top of the quilt front. I don't understand why that much excess is needed. I am planning on doing machine quilting on a regular sewing machine. Is all that excess only needed for those long arm machines and I can do just a couple inches on each side if doing it on the sewing machine?
I am thinking of self binding it with the backing, so that would make sense to have the backing bigger, but why the batting?
Sorry for likely such a clueless question.
My quilt is 70" wide by 87 1/2" long.