China's trade surplus and foreign direct investment (FDI) explain only 30% of this.

ECONOMIST: Capital inflows to China

Dollars are pouring in from China's trade surplus, as well as from foreign direct investment and from speculators betting that China's currency will appreciate.

NPR: Chinese Cash Moves Could Roil Markets

The island runs a large trade surplus, and its foreign reserves are the world's fourth largest, behind China, Japan, and Russia.

FORBES: Taiwan

Many of China's exports are products that are made in China on contract for foreign companies, he said, so the U.S. shouldn't criticize China for running a big trade surplus.

WSJ: Tech Supply Chain Exposes Limits of Trade Metrics