A western saddle spreads weight out over a larger area - something like 150+ % of the area. The white padded one is our Abetta, which has the smallest tree of any western saddle I've seen. The yellow is our Circle Y, and the tan in my Bates jump saddle:
Here is the difference between the actual trees:
If the behavior is significantly worse with the English saddle, then it may be creating too much pressure in terms of PSI on the muscle tissue and causing pain.
I have two English saddles. I've been swapping back and forth between them lately. I feel better with the jump saddle. However, the AP version has wider panels to distribute my weight, and a wider channel down the center to keep weight away from the horse's backbone. I don't jump, so I don't NEED the jump saddle. I just like it better. My horse doesn't act up in it, but I suspect she moves more freely and more relaxed in the AP saddle.
I plan on riding her with both over the next week to see which one works best for us as a team. A saddle can 'fit' and still not be 'right'.
The two English saddles...both Bates Caprilli, one is CC & one is AP:
The rear part of the panel of the AP saddle is 5 inches instead of 4.5 inches wide, so a 10% increase. The channel width, at its narrowest, is 3.25 vs 2.25, so a 44% increase. Those are not huge differences, but are something I need to consider before deciding which will be my go-to English saddle (I've had both for 3+ years, BTW).