It seems when talking about the first world war I usually hear huge numbers such hundreds of thousands or even millions thrown around on multiple fronts with nations gather huge sums of population of soldiers to fight the war. There is books talking about how marching column of German soldiers took almost days to walk through one village with logistical carriages going for miles upon miles as well right behind the army. Why was this war able to support such large numbers and huge scale fronts compared to the era of Napoleon barley 100 years prior? He seemed to have just as much territory and population size for recruitment not to mention the coalitions but neither not able to achieve anywhere close to the army sizes of WW1.
The late, great military historian John Terraine contextualised the First World War as the one of the three "wars of the Industrial Revolution" (the other two being the American Civil War and the Second World War). The arrival of industrialisation in the developed world provided the infrastructure for mass armies to be mobilised at short notice, rapidly transported to the frontline, and sustained in the field for an extended period. This could be done by widespread use of the railway, production lines making the goods capable of maintaining huge field armies, and factories churning out weapons and munitions en mass for years on end.
While the industrial revolution had begun in Britain by the time of the Napoleonic Wars, the nature of European societies as a whole was primarily agrarian, and they did not yet have the military/industrial capacity to fight a truly modern war in the style of 1914-1918 (a war that involved multi-million man armies backed by copious amounts of firepower). This new "modern" form of warfare first asserted itself in the American Civil War, as typified by Grant's Overland Campaign. It reached its apogee in Western Europe during WWI, and returned again with apocalyptic aggression on the Eastern Front of WWII.
See John Terraine's "The Smoke and the Fire: Myths and Anti-Myths of War 1861-1945"