Hmmmm, try basting around the edge of the quilt before adding the binding. by that i mean using a long stitch length and sewing near the edge by machine. then when you attach your folded 2.5 inch binding you can get it to fold over the edges easier. I tend to leave 1/4 or less of batting beyond the quilt top edge to help fill the binding.
try making something small and see how you do on the binding then.