The more I'm reading about historical events, the more doubts I have about various things, especially concerning size of ancient armies. Recently I stumbled on Wiki article about Battle of Thermopylae, and dubiometer gone off the scale when I saw estimates of Persian army. Hungreds of thousands?

"No real consensus exists; even the most recent estimates by academics vary between 120,000 and 300,000. As Holland puts it, "in short...we will never know.""

It seems to me that it just can't be right, even the lowest counts. How the army of this sheer size would be fed this far from home?

Wiki gives estimates of 300,000–500,000 for a full Persian invasion force. I tried to thought about it rationally. As far as I know, cavalry was an important part of Persian army. But even if ratio was something like 30 infantryman to 1 cavalry, it's still 10000 horses, who need to be fed at least 10-15 kg of food everyday, and drink, like, A LOT of water. Forgetting about horses, presumedly it's still hungreds of thousands of people who are not working, they need to be supported by state, so there's a need in a strong economy. If we put those numbers into perspective, they are roughly comparable with Napoleonic era, but nations of that era were far more advanced in logistics and medicine, and undoubtedly more productive at home. I find it impossible to believe that Persians managed to assemble army similar to Napoleon, but 2300 years before Napoleon.

Out of interest I've checked later conflicts in the region. Battles of Peloponnesian War are count in hungreds or thousands, which seems pretty reasonable to me. Then there was campaign of Alexander the Great, who presumedly assembled about 50 thousands man - a very big figure, but not as ridiculous. I have my doubts that Persia managed to create an army tenth of this size a century ago - and ultimately failed.

So, to the quick of the matter. If we're forgetting about ancient accounts, which in my understanding are tending to overestimate the figures, after all researches, can we now say that Persian army was really that big?

And a follow up question. Does modern historians take into account circumstantial evidence about, for example, size of ancient armies? Like, counting how much average soldier and horse would eat, researching the landscape where could they eat, or how a country can actually support all those troops. And can this method be actually used in historiography, or it couldn't be more than just a series of wild guesses?