A year of warfare.

The army met at Plataea could've been sent elsewhere during the Battle of Thermopylae and returned during the year of battle. It's not like the entire Persian army was present every single charge. It is mentioned Xerxes is a self-stylized God and he has conquered many, many lands, getting exotic animals like massive rhinos and elephants. It's not hard to imagine the army you're seeing then is part of it. There may be other Persians on the way to the battle and all you see then is the initial confrontation of that specific army and the Greek army.

All you can do is speculate, since it is a fictional account of Plataea with less than a minute of anything shown in it. I was posting the info earlier in case anyone was interested in what happened between the gaps in the movie in other places, or what happened after the movie. It wasn't specifically designed toward you, even if I did quote you.