I think it might be worth elaborating on 'they won't give representation'.
If Japan gives China representation equal to them, or not less than 1/10th as Japan, there will be more Chinese votes than Japanese. If you are envisioning a historically accurate system, replace voters with power because the Japanese empire was not a democracy, if you want equal representation then you mean they have equal power. They may use it to deny representation to Japan. They may use it to follow industrial and economic policies that move industry to China. They may use it to create a military full of Chinese soldiers sailors pilots, and lead by Chinese officers. They may use this vote to leave the Empire, perhaps after doing those things, perhaps immediately, or even never, instead keeping Japan as a colony.
Japanese voters and politicians or whoever holds the power in your theoretical system, knows this too. They know that making Chinese people equal to Japanese people means they will be put at their mercy and they have not exactly given them reason to be merciful, additionally they would find the idea of equality insulting.