Herodotus is a problem, but he is also one of the only glimpses we have into the non-Greek world before Xenophon shows up. I wouldn't believe all that much that he says as he is prone to exaggeration and is not a first hand observer much of the time, however there are likely grains of truth in just about everything he says, we just have to sift through and pick out the facts from the fiction.

We all know the line that Persians are taught three things from youth: to shoot a bow, to ride a horse, and to speak the truth. Well this probably applies only to the Persian nobility first off as most people would not own horses. As such, it is not necessarily true that the general levy (i.e. Persian Archers/Archer Spearmen) would be trained in using the bow any more than a western levy of Celtic Archers. Obviously there was a tradition of archery in Persia that did not exist in the West so bows were a more prevalent weapon in warfare, therefore the Persians come in larger sizes. Also, they had access to superior bows and therefore they have better range. However, it does not necessarily make them any more accurate with that weapon as individuals.

The Heavy Persian Archers may be a more professional fighting force though I am unsure of this and would defer to others. However, they are already represented as such in game so no problems there.