Originally Posted by LittleZeasel
But that brings me to the same question that was posed somewhere above: A horse's spine seems to be even front to back, why would you want to have it narrower on the back, than on the front?
On a physiological level, the spinal processes, and associated ligaments, are generally larger in the thoracic area. This is basically because they're under more stress - they must cope with a moving shoulder and ribcage plus the weight of major vital organs inside it. A wider channel here helps stop the saddle interfering with the ligaments either side of the spine.
If you press gently but firmly along either side of the spine it's possible to feel the ligaments (but try not to prod too hard - some horses don't like it!), and as you move towards the tail you should notice a reduction in size.
As as channel width is concerned, it seems to me to be a mish-mash of design ideas. Saddles originally designed for heavier types often (but not always) have a wider channel than those for TBs and warmbloods, but styles that prove popular with riders are sometimes used across tree-types with little thought for panel changes.
Saddle makers, especially the bigger firms, are leather workers and businessmen, not necessarily horse people. Sad but true.