It isn't a question of whether China would win, it is the certainty that Taiwan would lose. And regardless, that isn't the reason why large empires are unstable. Through history big countries tend to win but large agglomerations tend to dissolve for economic concerns. That is what happened in the largest imperial dissolutions in history (British, Mongols) which weren't because of a defeat by an external power or because they had any trouble conquering small powers. Or the most recent with things like the USSR.

Large empires tend to lose through military victories and bad economic strategy.