Well one could think they could field far larger numbers, however i am not sure... as i said their numbers increase slowly.Their general population really consists of NO old people i agree , and very few children... so the bulk of population COULD fight, in theory, for some time, except that still people need to grow food.But these would be Lmost inexpandable... once their dead, they will not be able to be replaced with a new generation for millenia.
Maybe they could have fielded a larger army... had they had more time to prepare... , but would they have really wanted to?