Not sure if I can explain this clearly...I've done this on a couple quilts. One of them, my sister had pieced the back, but it was so nice on the back, I didn't want to trim any of it, so I layered it...right sides together and batting on top...stitched the sides together and turned it. Because the back was a bit bigger, it wrapped around the sides when I turned it. I just layed it flat, made it even on both sides, pinned the edges and loaded it on my long-arm frame. When it was done, it had to only be bound on the top and bottom because the sides were already done. I quilted it all the way to the edges and it turned out really well, nothing wasted and it was completely reversible.
It's a little fussy to quilt it on the big frame, but it was fun and she didn't have to cut into her design to finish it. Before I had the big machine, I always used the pillowcase method and tied the quilt with yarn.