Think of it this way: The population of China (1,306,313,812 people) and the population of Finland (5,223,442) are quite different. The former is about 250 times higher than the latter. Obviously one is a lot more populated than the other. Compare these, if you will, to the Imperium and the Eldar civilization. Could each of these countries fill a shopping mall? Could they fill a football stadium? When you witness a large gathering in either China or Finland, you can't tell that there's a huge population difference there at a glance because that's not where you'd see it. If China had to put some tanks and maybe 100 soldiers in a theater of war, they could do it, but so could Finland. There's no difference there.
Over time, if China had to field that same force again and again, they would be unlikely to run out of trained soldiers. One battle after another would be staffed with hardened personnel and China would never have to worry about putting anything but their best people into the fray. If Finland had to do the same thing, it'd be different. After several battles, Finland's resources to field fully trained soldiers would diminish and wane because they just wouldn't have enough human bodies to serve as fighters. They could still put a full force on the field, but it would have to be mixed wth fewer "true" soldiers, and more militia conscripts/volunteers. They'd have to pad the ranks with citizens who weren't quite as prepared for battle as the Chinese.
Now just do some adjustment of scale for populations that are into the billions and trillions and it's a similar thing for the Eldar. They would not fail to put enough bodies on the field of war, but their access to hardened warriors would dwindle. This is where Guardians, drawn from the general population of Eldar, would enter the scene and serve alongside their more seasoned brethren (and, uh... sisteren).
Throughout history it's been done many, many times in countless military situaions: Civilizations of limited numbers will draw from the general populace to meet a military challenge in full force.