It looks like the borders are where the extra fabric is. Borders need to be sewn together and then the quilt edge measured and the body of the quilt pinned to match that measurement. Just sewing border after border around a quilt top ends up with this problem.
To fix you will either need to remove the borders, measure and pin and resew or make darts to take up the extra. Try pinning the extra fabric into darts in the back and see if it will lie flat. If it does and it looks okay, sew in the dart and cut away the extra fabric from the back. To disguise the extra seam when you quilt it, have a quilting line go over the extra seam.