My batting and backing are similar in size - both bigger than the quilt top. The reasons are 2-fold. a) If i don't baste the quilt sandwich as good as I should, there is plenty of added width & length incase the top shifts as I hand quilt the top. b) The extra is needed to attach to the frame and still allow me to quilt the top to the edges while still being attached to the quilt frame. I didn't allow plenty of extra and had to piece extra backing and batting to the back. Kinda bummed me out.