Numbers of troops in empires vs city states

[Shadowwalker]

im pretty sure a king wouldnt brag about crushing a rebellion of 1 000 people with 1 000 000 troops

Of course he would.
What's better to intimidate any potential rebel than to tell him that there are literally endless streams of soldiers coming for him if he starts anything?

Not to mention the tendency to boast about the own power level (both towards the people living at the same time and future generations):
"Look at me! I commanded millions of people! I was able to levy two hundreds of thousands in one single army - just teach those pesky rebels a lesson they wouldn't forget! That's how awesome my reign was!" ;)

but consider also that the persians were being supplied by a fleet of 500-900 ships. Thats a pretty massive fleet

True, but that fleet didn't just sail forth and back between the places where a food surplus was available and the greek peninsula. ;)
And even if they had done this - have a look at the amount of grain/water/etc a single soldier needs per day. Add in the food for the horses and other animals in an army and so on. And we didn't even mention all the non-food supplies needed....
The amount of supplies would have been astronomical.