Smaller seat sizes will fit a bigger range of horses, and if you couple with a fairly flat tree then it should be stable on flat backs, and can be slid back off a higher wither to compensate for the flat tree. And you have to really understand balancing a saddle with shims/pads.
Interesting question .
I had a medium wide ideal gp saddle years ago it fit so many horses I sold in a wild moment because it was scruffy and I hated jumping in it it it was a big mistake .
I have a Tekna dressage and a Thorowgood high wither dressage (both with changeable gullets although fitted with medium) and I find that with those 2 I can fit most riding school horses.