Let's look at it another way. Is it possible that the top is not stretching, but the rest of the quilt is pulling in instead? If she is pinning the top and not putting any side tension on the rest of the quilt, or stabilizing it to the correct shape before quilting, then the quilting stitches will draw the quilt in from the sides and all but the top (where it was stabilized by being attached to the leader) will be more narrow.