I really think it's the posture the saddle is setting you in, rather than the seat size.
I weigh between 200-210 any given time of year, and own 16-17" western saddles. Actually, the 16" fit me best, some 17" are too big for me and set me back too far on the horse. I have also ridden in 15"s but they are way too small for me.
You should fit in a 15" by most estimations but you could try different seat sizes just to see.
My pet peeve is saddles that have too much rise in the seat in front and force me against the cantle. That will kill your butt and tail bone (but some people like it?).
I like a saddle more like a hard seat ranch saddle like CowChick posted pictures of. They seem to make your crotch carry part of the weight and I really feel balanced and like it. So you have crotch and seat bones as your contact points instead of seat bones and tail bone.
Long story short, try a different style of seat to see if it's more comfortable. I bet it's the seat style rather than the size.