I was trying to explain “The New Trade Theory” and doing a poor job of it. Fortunately, Krugman does a good job simplifying it here.

The new trade theory starts with the observation that while this explains a lot of world trade, it also misses a lot. France and Germany sell lots of stuff to each other, even though they have similar climates and resources; so do the United States and Canada. What’s that about?

The answer is that there are many goods that aren’t like wheat or bananas, but are instead like wide-bodied jet aircraft. There are only a few places in which wide-bodied jets are produced, because of the enormous economies of scale – you only want a couple of factories worldwide. Those factories have to be somewhere, and those countries that get the factories export jets, while everyone else imports them.

There’s more details in the article.