>>>This probably isn't important, but most of the people were not part of the Roman Empire. Like Middle-earth, most of the population of our early world (and still today) were to the east and south, more the east in our world. South the Empire were millions of Africans, and east of the Empire were millions upon millions in Persia, India, Mongolia, China, etc. And north of the Empire were millions of Germanics. The Roman Empire was the most powerful empire in ancient history, but Gengis Kahn later controlled an area that puts shame to Rome, and had many times the population, including everything from Mesopotamia to Russia to Korea to Indonesia and everything in between.
But thats just a little off the subject. I have a new idea which might explain why the reinforcements from southern Gondor were so small. I think that almost all of the professional, armed and trained soldiers were already at Minas Tirith. And already at Minas Tirith were most of the able bodied men at an age to fight. The small amount of reinforcements that came were just 'scraped off the bottom of the barrel', you might say. Perhaps they were manning the forts on the sea against a possible southern attack by the Corsairs, but many came north, leaving only skeleton crews there. And when Aragorn came south with his army of the dead, ALL the men that were left, who had not been willing or able to go, came with him.
What do you guys think of this theory, that all the real soldiers were already at Minas Tirith, and the small amounts of reinforcements that came were only what was left in the southern fiefs?