Sorry if this is worded badly, but when you look at maps of Europe throughout the centuries, France was more or less always the same size and had the same amount of land from the get-go, unlike other countries who were able to compete with France only after uniting their region as was the case with Germany or uniting their kingdoms together (i.e Spain and Great Britain)
Why was this the case? How/why was France united into one country from the start, and if not, who were they united by?
All I know was that there was a Frank Empire in the region before France itself, but that's basically it, so the history of France is rather muddy and new to me.
France has some of the best geography in Europe. It’s south is defended by the Perones mountains, it’s west with the Atlantic Ocean, north west by the English Channel, south east by the Mediterranean, the East by the alps, and the northeast being the only non-naturally defended spot, unless you count the Rhine river which was mighty enough to also act as Rome’s natural boundary with the German societies. Now that we have established that the area is very defended from external threats, we can also see that it is incredible flat inland, meaning that whatever group conquered one area could do a good job of mobilizing across the whole region with ease. This can be contrasted with Greece which was infamous for its city state societies being based on the incredibly mountainous landscape that made it near impossible for one group to mobilize across the whole country. Finally, France has good weather and soil. It’s almost entirely arable land and there is little snow during winter. No other part of Europe has this combination of factors allowing one group to be able to consolidate such a large area of land and hold onto it easily.
The premise of your question is that France was a unified country by the end of the Middle Ages. If we look at France in 1453, this was generally the case. However, this was only after centuries of struggle between its royal house and other powers. Notable examples are the conquest of Normandy in 1214, the Albigensian Crusade which saw the Provence incorporated into the royal domain, the conquests of Gascony and Brittany, and the extinction of the House of Burgundy, leading to seizure of its lands. If anything, France was more divided than England (which ruled great chunks of France) for most of the Medieval period.
To give this centuries long process a single reason would be overly simplistic. While French unification centers on the royal house, there were many other factors. Geography is one, but language another. In the case of the Albigensian Crusade, land seizure allowed for integration of the southern "langue d'oc" into "langue d'oui", ie, it unified French language through displacement. My point is, there are many events and developments which each require their own study to answer your question.