The 2 strongest greek city state Athen and Spartan, have a population of around 300K and 500k according to wikipedia. While the Achaemenid Empire have a population of 35 million.
It was also said that Greek agriculture was labor extensive due to soil quality, while the Achaemenid Empire controls one of the more fertile land in history.
Given these two facts, why was Greece able to resist Achaemenid Empire?
Was it because that the shortest route to conquer Greece is through the Aegean Sea, and Persian navies are not as good as Greece?
I answered this question in detail here. When we're talking about populations, the real question is how much of a state's population can be mobilised for war. The Persians were fighting an overseas campaign at a great distance from their heartland, which is logistically difficult, time-consuming and costly. Their army could not be too big for too long, or they would not be able to sustain it; meanwhile, they were only able to bring as many men as could be missed in the regular harvest cycle at home. The Greeks, meanwhile, were fighting close to home on very short lines of supply, and could just about afford to throw their entire adult male populations into the fight. The result was that they were able to match Persian numbers on the sea at Salamis and on land at the decisive battle of Plataia.