The primary reason for the quantitative difference between the two militaries was simply French did not have to adhere to any arms control measures following the end of WW1, unlike the Germans who were militarily crippled by the treaty of Versailles. As a result, where the French were simply able to build tanks, test out new weapons, and solidify their doctrine, the German army was forced to develop tanks in secret in cooperation with the Soviet Union. Only with Hitler's rise to power in 1933 did the German Army really start to break with Versailles, and begin to build new weapons systems that had previously been prohibited for them. Thus, the primary reason was that the French army, and by extension the BEF, simply had a head start over the Germans, and had more time to order, manufacture, and stockpile weaponry. Therefore, by the fall of France, the Germans really only had 7 years to catch up with the French, who had continued building weapons since the first World War. However, this catch up period did mean that France had large numbers of relatively obsolete tanks. Where the most obsolete tank the German army fielded in the early years of World War 2 was the Panzer 1, the French still used FT Renaults, the tank that had helped to carry them to victory in the last world war. As a result, although the French had more tanks than the Germans, they were not always new or effective.

An example of how impactful the treaty of Versailles as on weapons development can be seen in the field of rocketry. Being one of the only fields of emerging weaponry not prohibited by the treaty of Versailles, the Germans were able to research and develop rockets throughout the interwar period, which partially explains why they were so far ahead of other nations in their employment and use. Where rocketry was one of many avenues for research and development available to most other interwar nations, it was one of the only ones the Germans could pursue legally.

Additionally, the French military of the Second World War relied on conscription to achieve maintain numbers in the face of its vastly depressed birth rates following World War 1. They also had a colonial empire to draw off of. For example, French West Africa mobilized approximately 100,000 "Tiralleur Senégalais", of whom 75,000 served in Metropolitan France, which were raised following the introduction of new conscription laws in 1919.

It is also important to note that the French were also preparing for war. In fact, unlike the British, who's military spending remained quite low until the outbreak of the war, French military spending remained robust throughout the inter war period, where they were had the second highest average annual defence expenditure as a percentage of GDP after the Japanese.

There is also a point to be made that strength is subjective, and that numbers alone do not necessarily make one military "stronger" than the other. The French doctrine was largely based around the concept of "Methodical Battle", which was an inherently offensive doctrine based on the lessons of the First World War, namely that overwhelming artillery firepower was the most powerful battlefield weapon, and that all other weapons systems were to be used in support of the infantry. For instance, despite having one of the largest air forces in the world by 1940, the French doctrine dictated that bombers were only to be used to strike what the artillery couldn't. In contrast, the Germans employed their traditional military doctrine of Bewegungskrieg, or war of movement, which was buttressed by many new doctrinal ideas developed by Hans von Seekt, Ernst Volkheim, and Heinz Guderian, and new ideas regarding close air support developed by Walter Wever and Wolfram Freiherr von Richtofen. During the fall of France, the Germans always worked hard to ensure that whenever they engaged French Armour, they always outnumbered their opponents, even if German tanks were less numerous than allied tanks. Moreover, due to fears that the Germans would intercept radio communications, the French military relied heavily on couriers throughout the campaign, which made the French military slow, unwieldy, and unable to respond to rapid changes in the military situation.

TLDR: The French had a large amount of obsolete equipment, had access to a massive colonial empire, and continued to fund their military during the interwar period, while the Germans were locked at 100,000 tankless men until 1933. The French also had a rigid doctrine that didn't employ its numerical superiority effectively.