that’s not quite how it (always) works.
Seat size is firstly about your actual seat size. But, if that model in a 16.5" seat technically fits your butt, but because of a longer thigh it forces your knees too far forward with an appropriately adjusted leather, than going to a 17" may be all the room you need to move back a little and still be centered in the seat.
This is also why many models have a standard, a more forward flap (and sometimes multiple degrees of forward.
Seat size has everything to do with butt size, it’s the only thing in the seat.
BUT also, the flap degree of forwardness has everything to do with an average femur length for that butt size. Someone with my butt size and femur will fit nicely in the same seat as someone with my butt size but a longer, or shorter femur, with our feet out of the stirrups.
However, the longer-femured person will either need a 1/2"-1" bigger seat to be able to move back and still be centered in the seat, OR a more forward flap. Usually it’s easier to go up a seat size. If they are super long in the femur, they should get a more forward flap and not a seat size 2" bigger.
The shorter-femured person should not move to a smaller seat size or they will be sitting too much on the back half of the saddle. You can go bigger, to some point, and still be effective, but you should never go smaller such that you’re center of weight is behind the center of the seat. At best that compromises your ability to stay centered in the seat, and at worst it means you are sitting “on” the cantle and putting undue pressure at that point of the horse’s back. They may need a slightly shorter flap, or just deal with a bit more in front than ideal, which is more acceptable than having a knee hanging over the front edge.
It also matters whether the seat is deeper and more U-shaped, or flatter with that U having a flat center. A 17" seat will fit larger with a flat center, smaller with a U shpae