Ancient army numbers

[MD]

Apropos Thucydides, he has an interesting quote in Book 6.33, in which Hermocrates from Syracuse claims that invaders can never come in greater numbers than the inhabitants of a country and even uses the Persian wars as example. Wouldn't that sound completely ridiculous if the Persian did indeed came with hundreds of thousands men to Greece and the Carthaginians with 300.000 to Sicily just a few decades earlier?

Few indeed have been the large armaments, either Hellenic or barbarian, that have gone far from home and been successful. They cannot be more numerous than the people of the country and their neighbours, all of whom fear leagues together; and if they miscarry for want of supplies in a foreign land, to those against whom their plans were laid none the less they leave renown, although they may themselves have been the main cause of their own discomfort. Thus these very Athenians rose by the defeat of the Mede, in a great measure due to accidental causes, from the mere fact that Athens had been the object of his attack; and this may very well be the case with us also.