1.5 million is a grossly exaggerated number, even if it includes soldiers, camp followers, slaves, etc. That kind of invasion force would have easily consumed all available food and resources between the Hellespont and Attica in a ridiculously short period of time. Food and fresh water aside an invasion force of that magnitude would have also been highly vulnerable to disease and the attrition rates would have been horrendous. It's true that Xerxes' invasion force was dependent on his Egyptian fleet and its ability to resupply them but if 1.5 million is the actual number I find it hard to believe that the fleet had time to do anything other than ferrying supplies back and forth between Greece and ports in Asia and Egypt, let alone mount an offensive against the Athenian navy.

Let's look at it from the Persian perspective. Why would Xerxes, Lord of Asia, king of kings and all that, believe he would require such a massive army needed to subdue a collection of uncooperative city states? Such an invasion force would have stripped the Persian empire's territories of much of its troops and left them vulnerable to attack and raiding by barbarians and neighboring kingdoms. The lessons learned by the Persians with the Greek victory at Marathon told them that Greek infantry was extremely effective but they must have realized that it was their lack of cavalry on the field that day which cost them the battle. Several hundred thousand seems to be a much more reasonable figure with the actual army coming in around a hundred thousand or so, two at the most. To the average uneducated peasant or anyone uninitiated with the ways of war bearing witness to the march of a few hundred thousand troops and camp followers it's easy to see how one could invoke the term 'millions'.