But in both cases, if there were a wide reflective and hi-vis stripe across the rider's butt, the rider would be even more visible.
I also think there should be a wide reflective and hi-vis band all the way around the wrists on jackets, jerseys, etc.. Where the hands are is often the widest part of the bicycle. As you can see from your photos, the rider with the hi-vis arm warmers has much more visible arms than the other rider, which could give drivers a better idea of how wide the cyclist is.