I know nothing about saddleseat. I do know western trees are longer than English, typically extending well beyond the cantle.
Someone sitting with their rump against the cantle in the above tree is barely behind the midpoint of the tree. If they are using an "Old West" approach with their legs to the front:
then the rider's center of gravity is probably in the middle of the saddle tree. The extra length and width reduce the pressure per square inch, so there isn't much pressure on the loins of the horse.
With an English (or Australian) saddle, the tree ends at the cantle - almost in front of the cantle. It also isn't nearly as wide.
That can keep it off the loins, but it also means that riding 'on your pockets' would put most of the rider's weight on the far back end of the tree.
My technique for deciding if a given saddle is too long is to have my horse pick up a faster trot and then bounce along in the saddle. I don't approve of that as a riding technique, but it gives a worst case for the horse. If the saddle is too long, or if the tree is too flat front to back and bridges, then my horses let me know - ears back at the cue for a trot, or ears twisted back in a "what the hell is going on" expression while they slow to a light jog to ease their back.
The first western saddle I bought was OK for length, but the bars were too flat for the curve of my horses backs and they did NOT like trotting. We gave it to a friend who had a long, flat backed QH.
The Circle Y Arabian saddle we have now (Mojave?) is 26.5". It works well for my tallest horse (Mia) & OK for Trooper. It is way too long for little 13 hand Cowboy, who has a short back even for his height! My Aussie-style saddles (with an English tree) are 22 inches long IIRC. They fit my Arabian & Arabian-Appy, but I also need to ride them with a more forward seat. We don't have a saddle short enough for Cowboy. We take a Wintec white foam pad and place it where the saddle ought to be. He has slender shoulders, so we then put the Aussie saddle too far forward on the pad. Because they were bought for wider horses, it allows his shoulders to slide under the front of the saddle while the rear is 3 inches in front of the rear of the Wintec pad. In minutes he went from 'cinchy' and irritable to relaxed.
Just eyeballing the saddle in your top picture, there is no way it looks too long to me. How does she ride with it?