There is a cap on trade balance. Once you go -4x NW negative, you start getting 'tax'. At some point you reach 98% tax, so if someone is sending you 100 bushels you'd only receive 2. Unless you have GBP, then it caps at 15%
Putting a cap on positive TB is just plain stupid.
That said, he 'only' had +60mil TB (which was 10 times his original NW). The rest was gold he sent away whilst receiving soldiers.