This is counting the slave population (it actually was a pretty small fraction in most societies, except those powerful enough to have a strong slave influx from war).
The harvests were shitty. This level of mobilization was balanced by being really short-term, which was one of the weaknesses of non-state societies, because when war was long enough, militias of smaller settlements often just left the main army and went home for agricultural work. So even if they managed to beat back the Roman Army, Rome often won just by maintaining the pressure, until non-state armies (if you can apply the term) started dwindling, then forced the battle on favourable conditions.