As a European power, France had to put a lot of manpower and resources into the war. Over 1,500,000 Frenchmen died, military and civilian, and many more were wounded. That is a lot of people from a single generation of offspring.
The Western Front also mostly took place in France, and four years of trench warfare took its toll on the French countryside. Verdun, for instance, is still a mess of shellholes, trenches not fully buries, and undetonated shells that make it too dangerous of a place to live. Other places are still not completely rebuilt even today, and not for the purpose of historical preservation.
To make up for all of the farmland lost, France had to rely on their colonies and importing from other countries more than ever before. This cost a lot of money.
Fortunately for France, they won the war in the end. Unfortunately, their role in the Treaty of Versailles would make Germany very mad at them, which was a factor in the invasion of France early in WWII.