Large, long and square skirts on a short-backed horse makes for a poor combination and could well be about a person more caring for style and looks than the comfort of their horse. The essentials of Western tree construction are fairly simple. As a horse bends his body to turn he needs room for his shoulder to curve without contact by point of the bar. Again, skirt design and attachment to the tree can make a well-made tree a poor fitting saddle, but there are many tree makers who do not put enough flare in the bars. Without enough flare the bar will be digging into that shoulder.
Rocker is the curve of the bar from front to back.
A decent rocker is essential so that the bars do not bridge and therefore rest on single points front and back. This can create sore spots as the weight is borne on individual locations rather than spread along the length of the bar as it should. This is also a point to consider when riding a mule. Mules have backs that are much straighter than horses' backs. So a horse saddle with good rocker will actually be pretty distressing to a mule.