Although every Imperial City covered a vast area, the number of players it could accommodate was limited. During a siege, the China Server Alliance had most of its players unable to enter the Imperial City to fight and could only remain outside, undoubtedly wasting a great deal of manpower.
However, if the Japanese Server Alliance attacked two Imperial Cities at the same time, the China Server Alliance would undoubtedly be able to fully mobilize sufficient forces. This would create greater resistance and even casualties for the attackers, perhaps even completely halting the Japanese Server Alliance's offensive and preventing them from destroying any Imperial City.
Indeed, although the Japanese Server Alliance had more elite players than the China Server Alliance due to having more servers in its ranks, the number of players they could mobilize in a short period during a siege was not that great. Even with several Space Element Players using spatial skills and Mass Teleport Scrolls, they could not move that many players. At the very least, their mobilization efficiency was far lower than that of the China Server Alliance—the Imperial City's Teleportation Array could transport players very efficiently, not to mention that the China Server Alliance also had quite a few Space Element Players.
Even if they had far more players than the China Server Alliance, if they could not bring them all over during battle and could only mobilize a small fraction, those players naturally would not provide much help in the siege. This was especially true when attacking two Imperial Cities at once, as the China Server Alliance had a stronger ability to mobilize troops.
In short, even though the Japanese Server Alliance was clearly stronger overall than the China Server Alliance, they did not have the strength to capture two Imperial Cities simultaneously.
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