I have touched on this before, but I thought I would show more clearly how/why this can be an issue if the backing for your quilt is not larger than the top. Most quilters will request anywhere from 3-6 inches on EACH side of extra fabric. Here is why:
The extra fabric on the top and bottom of the backing are used to pin the material to the canvas rolled at each end of the longarm machine, ‘right’ side facing downward. After both ends are pinned to the canvas, the quilter rolls the fabric under so the top edge of the backing is within the quilting space. Then the quilter lays the batting on top of the backing, and lays the quilt top over the batting. There’s your quilt sandwich.
If the top of the quilt is the same size as the backing, your quilt will end up short, because the backing is pinned to the canvas, the backing is now about an inch shorter than the top.
The sides need extra material so we can ensure all the wrinkles (not everyone irons their backing material…) are pulled out and that when the quilter moves from one side to the other, that they don’t push or pull the fabric and create folds on the back. This is most likely to happen at the edges.
The way a quilter can get around that issue is to drive the longarm quilting machine with one hand and hold the edge of the material taught with the other, but this can affect the quilting quality.
So please, my quilt making friends, please leave extra material on the sides and ends of your quilt for the quilter of your choice. You will make her (or him) happy and you will end up with better results. Happy quilting!