I've found that the 100% mohair or alpaca cinches will have some stretch to them. The last mohairs I bought were 28" out of the box but after some use, they are now 30". I'm not sure if the "blended" cinches will stretch because I don't buy those.
As for your rigging, do you have a picture of your saddle from the side so we can see the position of the front cinch D ring? If not, you can look at the link below and it will give you an idea of which one you've got. Your Complete Guide to Saddle Rigging
Unfortunately, the full rigging, which is one of the most common ones, isn't the one that fits most horses. When saddling most normal built horses with a full rigging you run into one of two problems. If you set the saddle forward enough for the bars to be in the right place, just behind the shoulder blade, then the cinch is too close to the front legs. If you set the saddle back where the cinch is in the right position, then the saddle itself is too far back and you run the risk of soring a horse's back.
It can be such a pain trying to get a good saddle that fits your horse and works for the discipline you are wanting to do.