Hey,
I went with the 1.25" mainly for the width being smaller. As mentioned a couple of threads up, Manufacturing would dictate the minimum width. I think the 1.25" should be ~1" more narrow.
Interesting.. So the width isn't determined based on an ergonomic design, but rather based on the equipment and diameter of bar? Seems like there isn't much ergonomic design thought put into it, rather, "If you want 1.5" bars, your width has to be X."
Ergonomics were probably considered, but possibly limited by the equipment they had at the time or what equipment was available.