I don't know what the rule is for western saddles but for English ones it can't go past the last rib of the horse. That's what I was always told anyway, don't know if that's right though :)
The rule is the same for all saddles, since it is the tree that bears the weight, and it should not go past the last rib that is attached to the spine (there are floating ribs beyond that rib, but they do not count).
That Western saddle , to me, looks too tight in the shoulders. I didnt' get a good look at the English saddle, but if you like, you might consider doing some photos of the saddles ON your horse, NO pad underneath , from front , angle, side, rear. Good lighting, horse on flat ground and squared up. Then one could get a good critique.
I think the seat of that western saddle is too small for you, too.
Waht measurement is it? I think you would need a 16.5 , perhaps.
It has to do with the length of the femur, too. So , how tall are you?
And the boobs , big or not, are hard to change, short of a scalpel. They will go down if you lose weight. I know the feeling, I am bigger than I'd like to be, too.