I fit into the multiple horses with just one saddle. My Ortho Flex fits Tennessee, who's 17hh and weighs 1300 lbs as well as Blade who's maybe 15hh and 900 lbs. So I can't complain.
English Saddles are a little more forgiving to fit. I can't imagine a horse my saddle wouldn't fit... and the gullet is exchangeable for a narrower or wider one if need be.
I think Western saddles are more particular because they are more rigid, and longer. Lots more places for them to not fit.