The largest container vessels afloat carry several times what the biggest ships of a generation ago could manage. The growth follows a straightforward cost calculation, and the costs it creates fall somewhere else entirely.
Fuel and crew do not scale with capacity
The resistance a hull meets in water rises more slowly than the volume it encloses, so a larger ship burns proportionally less fuel for each container it carries at a given speed.
Crew size is close to fixed regardless of vessel size, since the number of people needed to navigate and maintain a ship does not double when the ship does.
Port fees, insurance and financing follow similar patterns, all of which push the cost per slot down as the ship grows, provided the ship sails reasonably full.
Alliances made the big ships fillable
A vessel that large is only economic when nearly full, and no single carrier reliably had enough cargo on one route to guarantee that on every sailing.
Carriers responded by forming alliances that pool cargo and share space across each other's ships, which spread the risk of an underfilled sailing across several operators.
That arrangement made ordering the next size up rational for each carrier individually, and the ordering has continued for two decades on the same logic.
Ports absorbed the consequences
A larger ship needs a deeper approach channel, longer berths and taller cranes with a wider reach, and those are public or quasi-public investments made years in advance.
Dredging is expensive, environmentally contested and must be repeated as sediment returns, and a channel deepened for one class of vessel becomes inadequate when the next class arrives.
Ports that decline to invest lose the direct calls, so the decision is effectively made for them by the shipping lines' order books.
The peaks arrive all at once
Ten thousand containers landing in a single call must be moved through the terminal, onto trucks or trains, and out through a road network sized for an average day.
The same berth handles far fewer, far larger arrivals, so the terminal alternates between idle capacity and severe congestion rather than working at a steady rate.
Inland infrastructure sees the same shape, which is why a single large discharge can back up a regional highway for a day.
The savings and the costs sit in different accounts
Carriers capture the lower cost per container directly, and competition passes some of it to shippers as lower freight rates over time.
The dredging, the cranes, the road congestion and the resilience risk of concentrating so much cargo in one hull are borne by ports, taxpayers and cargo owners.
Whether the vessels are efficient depends heavily on where the boundary of the calculation is drawn, which is why the debate has never settled.