The last time I had a panto done, it required that she go over the side a good bit to do the design properly. Could this be what happened?

I am a Longarm Quilter. I would not keep any trimmings from a customers quilt, BUT:
If your backing fabric was not cut straight and square your quilter may have needed to trim it to square it up. You can't load it if the top and bottom edges are not straight and parallel to each other. If you did there would be uneven tension on the fabric.
I also try to load the top closer to the top edge of the backing so there is more of a safety net at the bottom. This may have accounted for the single trimming she did return,it would have come from the bottom.
Also remember that the finished dimension of the quilt will be less than the original dimensions. The quilting alone can cause the dimensions to 'shrink' a few %. This effect would be equal in the top and back but if you just measured the backing before and after the quilting you should expect to see a few inches 'disappear.
Most importantly talk to your quilter. He or she wants you to be satisfied with the results.

I agree with Bellaboo.She should understand that you need the scraps so to speak to finish it,without having to buy more.

I always get my extra back...I have just started asking my LA'er to trim the quilt because I've had king size quilts that I can't trim on my little cutting table...I pay extra for them to trim them and they are done very close...I have also paid extra to have them sew my binding on the front (I hand stitch to back). I have only used a total of 4 LA'ers...2 in Wisconsin, 1 who always trimmed the quilts and returned the trimmings and the other who didn't trim unless asked (and charged extra) and returned the trimmings. I have now used 2 LA'ers in Arkansas with also the same results...I would definitely request my trimmings back....I usually don't bind with the extra but it does get used and the extra batting is ALWAYS used, ie mug rugs, coasters, etc.

I would just send her an email. Explaining you plan to use the fabric for the binding. Drop the fabric off or mail it to you.
If you take anymore quilts to her I would tell her up front not to trim your quilt.
My LAQ never trims mine.

My LAer returns everything not used. I just used some "leftover" batting in a tablerunner. It was the perfect length and width I needed!!

Have to jump in here---as most of you know, a backing needs to be at least 6" longer and 6" wider than the top for longarm mounting. When the backing is so big that there are more than about 4" on either side (extra at the top & bottom doesn't matter), I trim off the excess BEFORE mounting the quilt. It is extra work for your longarmer when the backing is way oversize, and some longarmers charge for trimming; the whole problem can be solved by sending the correct size backing with the top! BTW, when I do have to trim backing, the long narrow strip ALWAYS goes home with the completed top.

No she does not. It belongs to the quilter maker not the professional who does the quilting. When I got my finished quilt back I also got the extra backing and the extra batting.

I had a quilt done by a long arm quilter and she did not trim it. Ask for it back. If she threw it away, maybe you can find more at your local fabric store.

I had my first quilt long-armed last summer. He did a good job, but did not return the excess. I had sent a king size batt, 120 x 120 inches. and expected about 120 x 30 inches back. I called them, and it wasn't right away. I had to stew a little first LOL. They said they still had it. The next time I was there, I forgot about asking for it. So I called them and asked if it was still there, and it was. They had just had a big blow-out sale, so I was concerned. I did pick it up later. It was stuffed into a bag, so I had to smooth it all out and roll it up so I could use it again. I butt those ends together and use as much as I can in charity quilts, and in family quilts as well.

I make pillows with all the little itty bitty ends and pieces of batting. I have a box under my table near the sewing machine, and whenever I have a little piece of batting in my hand, it can go easily into the box. Then once a year or so, I sew up a pillow ticking and stuff it. My local food pantry gives them away.