Actually, at the best of times, Gondor was the dominant power in Middle-earth, but that ascendancy had long since passed by the time of the War of the Ring.

: Also (as Tolkien illustrated in RotK) they seemed to rely
: heavily on irregulers and local militia (where Rome relied on
: heavily trained legions).

Er, no, Tolkien doesn't illustrate this at all. Martin has been arguing this is what they were doing, but the texts don't show that at all. Very few irregulars (perhaps no more than a couple hundred) are mentioned in THE RETURN OF THE KING.

: Just some things one should take into account when trying to
: determine the populations IMHO.

What I think is more credible here is the fact that Gondor was on its last legs. It was in a fight for its very existence. The Parthian campaign and even Adrianople were not similar conflicts. Adrianople resulted from an arrogant emperor's mishandling of a Gothic rebellion (and only part of the Goths were involved). Parthia was just a stupid attempt at conquest, IIRC.

Gondor had mobilized all its forces for war at the end of the Third Age. Every soldier was expected to see battle. Maybe half of them did see battle. We have no idea of how many men were lost at Pelargir. We only know that by the time Aragorn arrived there the Corsairs were in control of the city.

Now, Legolas does say, "and soon there was a great gathering of men out of Lebennin and the Ethir, and Angbor of Lamedon came up with all the horsemen that he could muster."

This statement is rather vague, but these men, if armed and armored for war, could not simply the local peasants leaving the farm for love of the Heir of Isildur. Whether militia or regular forces they would have to have been trained and mustered for some length of time.

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