This just establishes that large countries can split into small countries regardless of the economic consequences. But it tells us nothing about whether the division of large countries actually does increase barriers to free trade.

This suggests that the division of a large country into several smaller ones would strengthen the world economy. But it doesn’t prove that such a division will not increase barriers to free trade. The economy could still be stronger despite increased barriers to free trade.

Contrapositive: Countries that don’t think of themselves as economically self-sufficient don’t impose barriers to free trade. This proves that the division of countries doesn’t increase barriers to free trade, since small countries don’t think of themselves as economically self-sufficient.

The fact that barriers to free trade harm the economy doesn’t tell us anything about whether the division of large countries into small ones increases barriers to free trade. (D) just tells us that if such division does increase barriers to free trade, it would likely harm the economy.

Whether large or small countries are more economically self-sufficient, we know that small countries don’t view themselves as self-sufficient. We still need to establish that countries that don’t view themselves as economically self-sufficient don’t raise barriers to free trade.