Maybe it has something to do with how wide or narrow the twist of the saddle is? I broke my tailbone 3 yrs ago and have to be careful of how wide the twist is on my saddle or it puts me back right on the tip of my tailbone and boy does it ache for the next couple of days after a long ride!
I'm 5'6", 135 and ride a 15" western and a 17.5" dressage saddle just fine. I've actually been up to 150 and still didn't need to change seats. The 16" seem to make me feel like I have too much room between me and the pommel, but only by a little, so I prefer the 15". I also like my western saddles to either have at least a 4" cantle and prefer a 5" for hiney support - which tends to spare my lower back after long rides as well. In any of the sizes or styles, I prefer more of the equestrian type seat with my legs underneath me, even on my gaited horses (when most people tend to ride with their legs a little more in front).
All of these things combined work really well for me and I can condition for endurance all day long and be no worse for wear. But the saddle twist has to be more narrow for me to survive.
Like everyone else said already, it's really dependent on each saddle. I'm 5'6" and 160. My daily driver/training saddle is a 15.5" that my mom had made almost 30 years ago (Crawley), reining saddle (Cook)is a 16, english 17.5 (Collegiate Ian Millar) have several other saddles but those are what I ride the most.