Did you use your walking foot to attach the binding? The walker helps feed the quilt and binding strips evenly. This will help avoid stretching which it sounds like has happened to your quilt.
When I have a top ready, I bring the bottom edge to the top and make sure they measure the same width. Likewise, I bring the other two edges together and ensure the same size. Make any adjustments now. If you cannot trim, bind the long edge first by tugging a little on the binding strip to shorten. The feeddogs will ease in any excess fullness. Recheck the measurement if you have "tugged".
Hope this helps. Suzanne