I've made small quilts with backing only slightly bigger than the top, similar to your proportions, and they've turned out fine.
What basting method are you planning on doing, though? I think spray basting keeps everything lined up the best, with the least likelihood of running short on the backing after quilting. Be sure to read some of the tutorials on how to baste with basting spray. With a backing such as yours, you want to be sure to get the top centered accurately. One of the reasons extra fabric for the backing is recommended is so that, if you are off a bit on centering, you will still have all 3 layers in the quilting.
Do you plan to machine quilt? If so, I highly recommend heavily starching the backing and top before layering. For a small quilt, several layers of spray starch on each would be enough. (I usually use a very heavy starch solution on backing fabrics.) Starch stabilizes the top and backing so they are much less likely to stretch or distort while you are quilting.