Yeah, well, I'm speaking from similar experience to the OP. I have a VERY long femur and am tall and thin (and used to be uber skinny and no butt). She needs a saddle that fits her backside FIRST. And nowadays, you can more easily find that and then get a longer flap.
You very well may be speaking from experience. I would say what works for one person does not always work for another. It is not *just* about the size of your bum, or the length of your femur. It is also about how your femur comes out of your hip socket, and the shape, angle, and size of your pelvis.
I'm not saying that in certain saddles OP may not be more comfortable in a smaller seat. But she easily may be more comfortable in a larger size depending on her conformation and the particular model saddle. I watched the videos and I did not come away from that thinking that she was massively swimming in either of the saddles. In my very amateur opinion, when you are shopping for yourself, you should start with a saddle that works for your leg, and then go up or down in the seat size for your backside. You can't make a saddle that is absolutely wrong for your leg work no matter how small you make the seat.