He's saying that if you add up a bunch of negative net worths, eventually the sum is going to be very negative, even if each individual number was small. In par

He's saying that if you add up a bunch of negative net worths, eventually the sum is going to be very negative, even if each individual number was small. In particular, he's talking about the case where each of those numbers that you add up is individually smaller than your personal debt, but their sum exceeds your personal debt. Therefore, it looks like you are wealthier than all of those people combined (since your worth exceeds the sum of all theirs!).