I have seen that brand of saddle before, while American Made, the quality of the leather is not the best.
The Leather is very dry and stiff, it will need many weeks or months of oiling to be pliable and then the skirts may fit better in the back.
The fit of the bars is of course the key, it is hard to tell by those pics, but seems to fit in the front.
Have you run your fingers between your horses back and the bars of the saddle from front to back to see how the contact of the actual tree is?
If the bars are not making good contact at any point with your horse, then you have a bad fit. Like I mentioned lots of oiling will make the skirts softer and contour to the horse better.
May all your Trails be happy and safe ones