The reason, say the shipping lines: better economies of scale. It costs less to send a string of three or four big ships across the oceans instead of a larger fleet of smaller vessels. The new ships also will have more efficient engines and lower emissions, which means lower fuel costs as well.

Danish shipping giant AP Moller Maersk, for example, is spending $1.9 billion on 10 new ships that will carry 18,000 cargo containers each. The new ships will have a cargo capacity 16% greater than the world's biggest cargo ship currently afloat, the Emma Maersk. They will be more than 1,312 feet long, more than 193 feet wide and will stand 239 feet tall.

The new ULCS, or Ultra Large Container Ships, could hold an NFL football field, a standard NHL hockey rink and an NBA basketball court, laid end to end, and still have room to spare.

Maersk is not alone in the shipbuilding binge. AXS Alphaliner, the Paris-based maritime research firm, released a report Wednesday that said: "It appears that the container carriers’ answer to the challenges of sustainable shipping and the reduction of emissions is to build ever-larger ships. Compared to a decade ago, the average container ship size has doubled."

To give a sense of the scale of this transition, Alphaliner says that 48% of all new ships built in the coming years will be able to carry 10,000 or more cargo containers. Just five years ago, the world's largest container ship, the Gudrun Maersk, had a capacity of 9,500 containers.

Maersk said the new ships will help it achieve its goals of reducing costs and lowering emissions, while hopefully impressing customers.

"It is not only a top priority for us, but also for our customers, who depend on us in their supply chain, and also for a growing number of consumers who base their purchasing decisions on this type of information," said Maersk Line chief executive Eivind Kolding.