Weight distribution

So I have had an inkling that maybe its not so much my particular saddle that Ice doesn't like, but the type of saddle itself. I ride western, so I have a pretty substantial 25 pound all leather saddle. Its light compared to others that I've seen, but considering for the first ten years of his life, he never carried more than 2 pounds of tack, if that, it could be causing him some discomfort. In addition, he's got a narrow frame and a long back, not very good for carrying a western saddle at all. My trainer, who rides English commented on some youtube videos I sent her that he seemed uncomfortable in a western saddle that wasn't mine. Then when we tried my saddle on him, he blew up trying to get it off. This was just on a lunge line at a trot.

So, my first thought is "okay, new saddle", but then my trainer got on him in her little postage stamp of a saddle, perfectly light and airy, however crappy the rest of him looked. I've heard that Western and English saddles distribute the riders weight differently, but I can't remember the specifics. So, how different are they, and do the riders carry themselves different in them (as in, is their center of balance different). I'm just wondering if maybe I should suck it up and but an English saddle or if I'm over analyzing the situation.