I had the same problem with a bear paw quilt set on point - the setting triangles had the bias on the outside, and the outer border waved like lettuce. I didn't understand what was going on, and I didn't measure the outer borders through the center - I just sewed and cut them to the right length.
Once I started quilting I realized I had a problem. I ended up making tucks in the border. Since I was hand quilting straight lines across the border, I handquilted the tucks down.
At the time I was devastated - the quilt was supposed to be for my sister-in-law, but I thought it was flawed and ruined forever, so I kept it. Now, I can hardly find the tucks! It helps that the border fabric was solid white, and the quilting thread was white to match.
You could try stay stitching the bias triangles, then reattaching the borders, taking the measurements through the center of the quilt. If you don't want to take the quilt apart, try basting the outer edge and see if that helps with the extra fullness.
I hope it works out for you.
Janet