The First World War was fought on a scale unprecedented in 19th century European warfare. One of the reasons for this was that it was a total war, fought by states mobilising all their resources to inflict absolute defeat on the enemy. Yet another was simply the volume of people entailed in this kind of total mobilisation by 1914.
Germany was able to mobilise around 11 million men, Britain and France 9 and 8 million, respectively. In the 19th century this simply would not have been feasible without causing the entire country to grind to a halt. This demographic expansion, combined with the material and technological developments which occurred in the same period, led to a staggering human cost on all sides.
British, French and to a lesser extent German populations were in effect even bigger than stated in the below graph due to their colonial Empires from which they could draw hundreds of thousands of additional soldiers.