France had 30 million inhabitants around 1800, and somehow only 40 million in 1914, and 65 million today. This tells how the country was already very much populated. Compare that to Britain : around 10 million in 1800, 30-40 million in 1914! This (lack of) demographic boom explains why France lost much of its status compared to its neighbours in the 19th Century, especially the British Empire and Germany.The french region was ridiculously populous compared to most other european areas during this period. Until the 19th century France was almost certainly the most populous european region when it was surpassed by Germany (which plays into the Franco-Prussian War). The Hundred Years War was a civil war between multiple sides of the Kingdom of France. The King of England of the time was descended of a French Duke that owned Anjou and Aquitaine and pressed a claim on England. It's like the Margrave of Brandenburg and King in Prussia situation. The guy was just one of many french dukes, but suddenly he's also the King of England. Shit -> Fan. Yet England was ruled from France by the Plantagenets more often than not, and treated more as a possession of French Lords rather than a realm in its own right. That would continue until a welshman won a civil war for the throne of England and the last pieces of France were reclaimed by France.
Agincourt is a bunch of BS btw. Archaeologists have looked at the area and they believe the english and french were very evenly matched at the battle. It was just an english fish story about how outnumbered they were and still won.
I suppose all medieval historians since the 15th Century have all been wrong then. What a shame, isn't it? (I'm curious to know to what study you are referring)