Follow up

Over the course of the war, the Romans would raise several armies of similar size to Cannae, and lose a few of them. How did this impact the job market/demographics of Rome? Did the huge drop in the male population lead to any major social changes?

So was WWI really not that extreme compared to ancient battles?

I would say that this is a great example of what can happen when numbers are quoted without context.

Battles in the ancient world (being eurocentric) were often fought by dense formations of troops as the majority of fighting was done with melee weapons, best employed in close formation. WW1 was fought with weaponry that forced a dispersion of forces, as tightly packed masses of men were an easy target for explosives, artillery, machine guns, or even a well drilled group of riflemen.

These past battles were often the climax of weeks or months of manoeuvre, and a war could be won or lost on a single battle or chain of battles. So whilst in one day you might have massive casualties, the weeks and months surrounding would be relatively calm, mostly involving the commanders attempting to gain the terrain advantage and skirmishing between the scouting forces of each army. It's also important to note that Cannae was an outlier, due to the Romans being encircled. Normally an army would have withdrawn or routed long before being fully destroyed. Current wisdom suggests that many of the casualties in ancient battles occurred after one force had been routed, rather than during the pitched battle.

By contrast, WW1 in Europe was fought on a front spanning from Switzerland to Belgium, from the Baltic to the Black sea, and on the Austro-Italian border. This ignores the fighting that took place elsewhere in the world, such as the territories of the Ottoman Empire. Battles would span days, weeks, or months, and subject the combatants to a long grinding attrition instead of a single burst of casualties. Whilst a single days casualties might be less than some ancient battles, the long attritional grind built up casualty numbers that are frankly terrifying. A quick glance at the numbers on Wiki gives a middle ground estimate of there being 700,000 casualties total at Verdun, during an 11 month campaign of attrition.

Looking at single day casualty figures for a single battle of WW1 (the Somme is where I assume those figures are from) belies the true scale, as unlike ancient warfare, there were multiple battles of the same scale occuring simultaneously across hundreds of miles of frontline, rather than one decisive meeting. A day's casualties might be beaten by the likes of cannae, but the casualties of a campaign in WW1 would utterly dwarf those of an ancient campaign.