The Second World War covered a far great part of the globe, but the Great War cost far more British and French soldiers lives. This was due mainly to the fact that initially in World War Two the German blitzkrieg quickly overran the allies. It was not until the Normandy invasion of June 6, 1944 that the allies got back into the game in Europe.
The biggest losers during the Second World War by far were the Russian army and civilians on all sides, the latter because of heavy air bombardment. The Second World War was mainly an industrial war, a battle of the production of ships, tanks, plane and oil refining. As civilians were critical to war production they were considered a prime target for carpet bombing of entire cities.
In contrast because tanks and planes had yet to reach their full potential, World War One, lacking mobility, became bogged down in relentless trench warfare. By the time millions of Americans reached Europe in 1918 both the allies and the Germans were badly used up. Thus the huge influx of Americans into the fray rappidly tipped the war in favor of the allies.
Still, America’s own Civil War of the 1860s killed more Americans soldiers than all the rests of her wars combined, and that was with a population of only a fraction of what it is today. Of course that is counting both sides — Union and Confederate.