As we all know saddle fit is important for our horses but it is also just as important for the rider, not just for their comfort but it also has an impact on saddle fit for their horse. If the saddle seat is too big for the rider it causes a number of issues for both horse and rider, it puts the riders seat too far behind the stirrup position which means they will ride behind their horses motion, reducing their balance and affecting their horses performance. It means the rider is not seated in the correct position to evenly distribute their weight over the bars of their saddle (even if they have an independant seat) creating pressure points on their horses back leading to stiffness and soreness. Whether a beginner or experienced rider, proper seat fit and riding position make a huge difference in riding technique and ability to accomplish goals.
It would be so much easier if the 'seat size' was the only factor impacting the size of the saddle but unfortunately seat size is complicated by ..if we're talking western saddles.. the angle and style of the fork or swell, the dish and slope of the cantle and the depth and angle of the seat itself all contribute to the room available for the rider so that's why we've got to sit in a saddle to truely know if it's the correct for us aswell as our horses :)