Ok ok, perhaps I was being too general, but if we didn’t think that large ponies had a bigger step than mediums, then why would we adjust the lines at all? Why not just have them all on a 12’ stride with no in and outs and you pick whatever number is suitable for your pony? That’s not how rated pony hunters are set up currently.
Of course you are correct that height isn’t the only factor for stride length. I had a small junior who could walk the lines and one who had to motor, and they were the same size. I concede that you could have a top end of the scale measure pony that can’t make the strides. The analogy I made to the small/large juniors is the same, though. Why else did that split occur, with age being a secondary split? Small/large split is based on the idea that odds are there are more horses over 16hh that can go slow on a 13’ stride line than ones under 16hh. But no, not every horse is suited for that division no matter what their height. I’ll add that my small junior with a big step was actually protested once and we had to re-measure (unless we put him in platform shoes he was never going to make a large measure, but we had to jump through all the hoops with the steward anyway).