You, along with many of people before you, missed one single point. Which defensive number superiority does not exist, as one simply rules show.
The point being, for the defender. You need to split your defense force so it would be able to defend all the borders within your country. otherwise your enemy can bypass undefended area and attack your defense from behind, flank or anywhere you are not expected to attack from. Which is how and why France lost that quick during the Battle of France in 1940, as they only focus their defense on Maginot Line but ignored the Ardennes.
If your country is appear a square shape, then you will need to defend the square equally at least (but can be weighted if there are BUND (Build Up Natural Defense) such as a river or cliff face or MMDL (Man Made Defense Line) Such as Maginot Line. Still, you need to position your troop and support element in that land to fend off possible attacks.
However, For an attackers, you only need to penetrate one line, and you will capture your objective. You can attack more than one line to draw off forces for the main attack, but for an attack, you do not need to attack all the defender defense.
Also, it's worth notice that for a continuous defense line, the attacker only need to attack a few point of the line, while the defender have to defend the whole length of the line to make the defensive works.
Now, what does that mean? This is a map of China.
View attachment 379146
You can see the Chinese coast line is not a straight line, also the internal border is not a straight line.
Now, let's say if I want to attack Beijing, and for this case, let's give some advantage to China and say I am going to attack via the coastline along Tianjin and Liaoning (in reality, i can attack from a further point and thus stretching the Chinese defense further, but in this case, we use a fix point)
For China to defend the city of Beijing, the first thing is very oblivious, you cannot fit 2 millions army, 1100 fighter craft all inside Beijing. That mean you will need to evenly distribute your force around the city and area surrounding Beijing. Which mean for China to successfully defend Beijing, China need to put force in or around the City of
1.)Liaoning Province
2.)Shandong Province
3.)Heike Province
4.)Beijing City
Let's say for argument sake, you only put force in these 4 provinces and city, you still diluted your force 4 times, just to protect one city. For the me, if I want to invade Beijing, I can land on any point at Liaoning to Shandong and then head to Heibe and then head to Beijing.
So in effect, even in this handicap scenario, China need to dilute her force 4 times, yet I can concentrate my force in one route. or two (I can land on both Shandong and Liaoning) and pin both defender down while i work my way into Beijing, and my force is unchanged, for example if I have 500 tanks and 400 fighter covering the landing on Shandong, i will have 500 tanks i can use on my Shandong offensive. However, if China have 2000 tanks and 1100 fighter for her disposal to defend Beijing, then according to force dilution, I am only facing 500 Chinese tank and 250-300 fighter that defend Shandong, given the defensive line is equally distributed (500 tanks for Liaoning, 500 tanks for Hebei and 500 tanks on Beijing). And it will take time to move defensive asset around, the time I can use to break thru your line, land reinforcement, or even engage a new offensive to further blogging down your troop concentration.
Now, imagine, I do not only have 1 objective, and I do not only have 1 route, that way I can further dilute the Chinese force where I kept my own force relatively intact, as I will always only have 1 spearhead, but China have to defend the whole line.