In the quilting process the pieced(top) will stretch ever so slightly, while the back, being primarily a whole cloth does not have that stretch. Cut a square of fab the same size as block and pull on both.....there will be more " play" in the pieced block......this is the reason L/A ask for larger back......and the more intense the quilting, the more the back is pulled in. So, although, I have never put a border on the back, nor have any of my clients, actually I would not attempt doing that for myself or others. I would suggest you make you center smaller and you border bigger...the
beauty of the quilt should be the pieced side.