Ancient army numbers

[Paralus]

There is no "leading his army down on the center" in the text, which would indeed point to your conclusion. The text reads "lead the troops against the center of the enemy" (ἄγειν τὸ στράτευμα κατὰ μέσον τὸ τῶν πολεμίων) with no other instruction which is much more neutral.

Yes, which is a "freer" construction of κατὰ μέσον = "down upon the middle". I have provided a more literal rendering in that the instruction is to lead the army (Clearchus' forces) down upon the middle which is where the Great King is situated. Xenophon even claims that Clearchus could see the "the compact body at the enemy's centre" which is a plain nonsense if the Great King, in his centre, is out beyond the rebel's left wing.

What you "dispute" is not the actual numbers (unless we are talking about really small armies of maybe 20-30,000 mean each) but the proposed analogy.

No, I dispute the numbers based on the fact that Clearchus could clearly discern the enemy's centre out beyond his force's own left and that he was instructed to march toward it. It is clear that the King's line did not hugely outflank that of Cyrus no matter Xenophon's claim. This is the source who, had the king's bastard brother arrived in time, would have us believe that Cyrus meant to take on 1,200,000 with his force.

An army of 30,000 or so was actually normal for the time. Why, in the Successor period and the following Hellenistic kingdoms do we never hear of such huge numbers? Why did Darius III feel it necessary to defend his empire at Issus with 600,000 but Antiochus III field only some 70,000? The answer is that the latter figure bears a close resembalnce to reality and the former is fiction.

Also, Cyrus does not lead his center against the king, Xenophon has him outside the battle serving as some kind of cavalry reserve.

That is not supported by the source material. Diodorus (14.23.5)states that

In the centre of the lines, it so happened, were stationed both the men who were contending for the kingship. Consequently, becoming aware of this fact, they made at each other, being eagerly desirous of deciding the issue of the battle by their own hands...

But perhaps you rely upon the eye witness Xenophon? He too confirms the fact:

And upon hearing this Cyrus said, “Well, I accept it, and so let it be.” After he had said these words he rode back to his own position...

And where was this position to which he returned?

...on the left was Ariaeus, Cyrus' lieutenant, with the rest of the barbarian army, and in the centre Cyrus and his horsemen, about six hundred in number.

So, having conversed with Xenophon, Cyrus returned to his position. Which position was in the centre of his line as Xenophon confirms again stating that barbarian generals always command from the centre.

Nothing in the source material supports the notion that Cyrus was outside the battle as a reserve cavalry force.

Xenophon justifies the decision of Clearchus not to accept Cyrus' proposal and break off the right wing because he was afraid of being surrounded from both flanks. (ἀλλ’ ὅμως ὁ Κλέαρχος οὐκ ἤθελεν ἀποσπάσαι ἀπὸ τοῦ ποταμοῦ τὸ δεξιὸν κέρας, φοβούμενος μὴ κυκλωθείη ἑκατέρωθεν). The text seems to be about his troops only which would require operating without allies on his flanks, that would mean outside the battle-line, although I guess that it could be argued that his fear is about the whole of Cyrus' army.

Clearchus had the protection of the Paphlagonian cavalry and light armed on his right. He clearly could see that the left would be outflanked (by the enemy centre which he somehow espied) and felt that if he marched away from the river his right end of the line too would be flanked.

As to the question of whether I think this oblique advance meant wheeling left (a la Mantinea 418), I raised this with David Thomas last September using that very example. He disagreed on the very understandable basis that, despite Diodorus, the Lacedaemonian component was far too small and to suggest their tactics (Mantinea) was more than a hoplite too far. He felt that the angled march towards the centre was meant.