I think you have a big misconception about france. They were a major player in world affairs pre-ww2, and continue to be a major player today, on par with England. Before the war france owned Syria and lebanon, most of West Africa, Madagascar, Indochina, and countless islands all around the world. France routinely interacts with its former and current subjects, same as the uk.

If you want to mark the decline of France, (decline isn't the right word), you want to start with the franco - prussian war. France was the greatest land power in Europe and was humiliated by the newly founded german nation. Germany was seen as a threat, the first real threat that France had seen in a very long time. The time period before the great war was less a decline of France and more of the rising of Germany.

WW1 simultaneously scarred and enlarged the French empire. Germany was humiliated, alsace Lorraine was taken back, France seized german African colonies, and they got Syria and lebanon for the cherry on top. Even during this time, french was still the international language of diplomacy, with the treaty of Versailles being in french and English, and the league of nations with 3 languages; french, English, and Spanish.

It's not until ww2 that we start to see a real decline in french prestige. Their surrender to the nazis was a massive blow. England would not suffer the same decline until shortly after ww2. But even the loss of their colonial empire couldn't make france irrelevant.

When the EU was created, France was the top dog, west Germany was number 2. England didn't join for another 16 years. When Germany reunited and became the most powerful eu state, France was always nipping at their heels, providing a balance of power in the system.

France did and has kept up with England. Their power never truly declined in the way that Spain or portugal did.

France declined for a variety of reasons, overcommitment to foreign wars leading to debt, leading to Revolution, political instability, and then the Napoleonic Wars which resulted in a system of alliances designed explicitly to contain then (the concert of Europe). All that, coupled with a declining birth rate and a lack of industrialisation when compared with England and Germany explains a lot.

Spain faltered earlier, with runaway inflation from the colonial silver trade, constant involvement in costly wars, failure to industrialise, etc.

The reasons for Britain's dominance were less about "territory" and more about the cost of fighting wars to maintain that territory vs the value extracted from it. The value multiplier of the Industrial Revolution plus Britain's trade dominance in India, Latin America and various other parts of the globe - as well as the fact Britain was afforded the geographic protection of the English channel explains at least part of their dominance. We can see the rise of America in similar terms despite the lack of an extensive "empire" in its traditional definition.

Tl;Dr: Industrial capacity is more important than square acreage.

Others have already pointed out your possible misconception about France's current and historical power.

I will just add one more thing. It's the english language. The two-punch of USA-UK made english de facto world lingua franca. And that both exaggerate and help spread the influence of UK across the world, while simultaneously diminishing (rightly or not) the achievements of french scientists and culture.

The US's global economic dominance can be attributed to the cost of WW1. Here is a good read on pre-WW1 economics Moving the global economic capitol of the world to Manhattan from London was a enormous blow to European dominance in the 20th century.

All things considered, France's modern military is enormous in comparison with the rest of Europe. It's one of the only European military's able to project force on a large scale due to aircraft carriers.