From what I can garner from light googling, the estimated citizen population of Sparta at the beginning of the 5th century BC was 20,000-35,000 (I have no idea if this is accurate, but they're the only figures I could find). Unless only males are counted as "citizens" then we must immediately half the number since only men would be fighting, leaving 10,000-17,500. If the lower figure is right, then the entire male citizen population of Sparta was at Plataea, and even if we take the largest estimate, it was still considerably more than half. And that is without considering the fact the presumably not all Spartiates were of fighting age or fit for combat (in which case the number Spartans at Plataea was greater than the lower estimates for the entire eligible population). There would also be factors like attrition to take into account.

The absurdly huge figures Herodotus provides for the Persian forces in the Greco-Persian wars have long since been proven a logistical impossibility, used in order to give the reader the impression that hopelessly outnumbered Greeks prevailed against an insurmountable colossus. That in mind, why would he overestimate the size of the Greek force? Based on his careful arithmetic and oddly precise final total for the inconceivably enormous Persian force, it doesn't seem on brand to just pull a nice big round number out of thin air for the Greeks, not to mention that exaggerating their numbers would run counter to the narrative he's trying to push.

Did ancient Greek city-states genuinely raise every single male citizen for war, keeping nothing in reserve? Surely this would come with all sorts of practical issues. Who would be running things at home or keeping order while all the male citizenry was gone? What, for example, would stop the underclass of Helots who comprised the bulk of the actual population from revolting while all their masters (and the entire military of the polity that enslaved them) was temporarily out of the picture?