Agreeing with all the above. I have added extra fabric to all sides of the backing when I was presented with a backing that was not large enough. Sometimes the backing would be just an inch or maybe 2 inches larger than the top. I like to use strips of muslin for this. I butt the muslin up to the sides of the backing, use a large zig zag stitch and whip them together. This way after the quilt is quilted I can snip off the muslin very easy and don't have to contend with a seam allowance along the edge of the backing.