I had the same problem with a binding that I did not too long ago. I wanted the binding to look wider on the quilt so I made the strips 3 inches. The problem is in the trimming of the quilt. If you have a wider binding strip then you have to compensate with how much of a batting border you leave on the quilt after quilting. That bit of batting border is what fills your binding. I hope that makes sense.
Personally, I would think about redoing the binding in a narrower width ( 2 1/2 or so) and then you won't have the gap. The next time you want a wider binding trim the quilt accordingly.
ETA: I think that you may have wrinkling or puckering once you wash the quilt if the binding is not filled. This is the reason that I suggest you remove it and redo it. The other option would be to remove the binding and then insert a strip of batting into the binding to fill it and then restitch it down. I think I may have read a post on this but I can't remember if it was on this board or another.