Eldar are so numerous on the battlefeild because of one key difference between them and the armies of other humanoid races: they ALL go to war. If an Imperial world goes to war, they will send out the Imp Guard, or maybee call upon some space marines, but the civilians stay at home and hope they dont get fusion-bombed. The same goes for tau, only the fire caste is going to fight. On the other hand, when an Eldar craftworld goes to war, those on the path of the warrior don their aspect armor, the avatar is awakened, the wraith troops are loaded with spirit stones, those who used to be on the path of the warior form Storm Guardian units, some seers join the warhost, and (here's the real reason) ALL the civilians are assigned to Guardian Defender squads. Sure, they don't all go to war at once, but the fact is that in a desperate situation, they could. Eldar feild huge numbers because they are feilding huge numbers of mediocre-ly trained civilians.
Ok, i'm done. Peace!