Rahul Oka, Agustin Fuentes, Susan Sheridan, Mark Golitko, Nam Kim, and I recently published a paper in PNAS looking at how demographics affects both the size of a war group and conflict causalities.
Here is the gist:
Many scholars have noted that in hunter-gather populations, the number of people involved in fighting can be as much as 40% of the population, but this percentage drops in big state-level societies.