Height, conformation, physical traits, and genetics are going to vary from region to region and herd to herd. That's because Mustangs aren't a homogenous breed at all, but rather a ragtag group of feral horses from all different backgrounds. Naturally the groups with more recent additions from domestic populations, or with significant draft or Thoroughbred blood, are going to be taller.
At that very reason is why it's quite a bad idea to breed Mustangs. There's no consistency, no "type," and nothing desirable about them as a whole. Yes, there are good individuals, and some are very fine horses, but you can't make any sort of generalizations about them. Many of them ARE of poor quality, quite honestly, and thus they are in very low demand. No one wants the ones the BLM currently has, which is why there's all the current brouhaha about slaughtering them. Making more is completely and utterly illogical.