In HOI2, China, (warlords included) has 114 provinces. This is a lot, but it’s far less than Western Russia (Barbarossa scenario 188 provinces.) The main problem is the province density. A province in HOI represents the area that one division can control, effectively deny another from operating in and come together to form a coherent defense In WW2, with good infrastructure and the right terrain, a single division could control a huge area and remain an intact fighting formation. Bad roads and infrastructure made this area smaller as divisional assets had to stay closer to be mutually supporting. Large local populations also made this area smaller as the division had to be more careful of partisans and had to guard more targets.
Bearing this in mind, the HOI map is not granular enough for China. Zhejiang province in Eastern China is represented by four HOI 2 provinces. This is less than Denmark despite being having over twice the size, 10 times more population and far more historic combat during the period.
Where this has real consequences is in defense. It's perfectly historical that there might 30+ Chinese divisions in an area the size of those 4 provinces. What's not historical is that the defenders could support each other over an area that large, which is what the superstack represents, divisions fighting together as a cohesive army. Two divisions in China could be 20 km apart as the crow flies but a week apart in travel time, yet those same divisions will instantly become a mutually supporting stack if their province is attacked. The province size makes Chinese defending superstacks possible.
Worse, the Japanese can advance as a single cohesive wave with no holes. Even with a million troops on the ground, they couldn't hope to maintain a coherent front and they didn't try. All they could effectively control were the cities and the rails between them with periodic sweeps into the vastly larger countryside. They even had a name for it. It was called “points and lines.” Whole Chinese armies, almost 500,000 nationalist troops and over a million communist ones were stationed behind Japanese lines when the war ended. With the current size Japanese strategy is just “invincible blob.” There is no risk of Japanese spearheads becoming dispersed and vulnerable to encirclement. There is no representation of how much of an obstruction even small amounts intervening terrain were to command and coordination once they got away from the rails. Hell, the provinces are too big to even represent the rails. Historically, the Japanese could and did win victories all across the front but still had to leave huge amounts of real estate unoccupied and full of Chinese troops.