Link isn't working for me unfortunately. Every other link I checked on the site does though.
Improved medicine/hygiene is definitely a huge factor in what percentage of people die in a war. I don't have any exact numbers but I know in many campaigns, especially in ancient times, more lives were lost to disease than steel.
Another big difference is the development of states and democracy. A king who believes he is ordained by god (and more importantly has subjects who believe the king is ordained by god) has far fewer qualms than most democratically elected leaders about sending large numbers of his citizenry into combat.
The world wars were a horrific confluence of factors where huge leaps in technology combined with fertile grounds for a war and things spiraled horribly out of control. I would expect that if the graphs are done by percentage that they'd be somewhat even up till the early 1800s, start a slow decline, ramp up for the world wars, then go back into a decline.