I am asking this because I remember reading about how the French had an abnormally small generation because of such heavy losses from WW1 and therefore did not have enough men in their military.This meant they had to play defensive by building the Maginot line for exampme. Why was this not the case for Germany who seemed to recover its manpower after suffering similar losses in WW1?
The fact is, France was suffering a population deficit compared to Germany even BEFORE WWI; WWI merely exacerbated it further. Germany suffered heavy losses in WWI, but was better able to comeback from these losses than France.
It's worth noting, however, that both sides invested in fortifications, the Siegfried Line and the Ostwall in Germany's case, the Maginot Line in France's case. Moreover, the Maginot Line didn't necessarily confine France to the defensive. The whole point of the line was to force the Germans to repeat the Schlieffen Plan and go through Belgium. This would mean that in order to get to France, the Germans would find themselves in conflict with Belgium, Britain and probably the Netherlands.
The French mobile forces would enter Belgium with the BEF, support the Belgians by stopping the Germans at the Dyle Line, and eventually go on the offensive, probably pushing into Germany. By that point, the Royal Navy would have Germany under blockade as in the First World War, while the British and French air forces would be bombing German industry.