Behind Germany's export success is an outsize global sales-promotion network that is helping to boost a trade surplus that has become a source of friction with its European and U.S. allies.

The country's already large surplus in trade and investment income rose by almost 18% in September compared with a year earlier—as new data showed the balances posted by other euro-zone countries weakened slightly.

The European Central Bank said Monday that the current-account surplus of the 17-country euro zone grew less than 4% to €13.7 billion ($18.5 billion) in September—which was less than Germany's surplus alone of €20 billion. The current account is the broadest measure of trade and investment income.

Such figures have put Germany on the spot for boosting its economy through foreign sales rather than domestic consumption. The U.S. Treasury last month said Germany's export-focused approach, without a similar effort to lift domestic demand, hampered Europe's recovery from its financial crisis, which in turn dragged on the world economy. The EU last week announced it would assess whether Germany's trade balance has breached economic targets for the 28-country bloc.

Yet there is one, sometimes underestimated, reason for Germany's export success: a low-profile but sophisticated global network of representatives built over decades to establish an export footprint for even the smallest German companies.