21 years ago I was in a Korean shipyard where the largest container ships of the day were being built. Their capacity was 6000 TEUs, one-third of the Mary Maersk. The captain of one of the ships (a 27 year old (!) German woman) told me most ports cannot handle the 6000 TEU vessels, so their line will be building mainly 4500 TEU ships. One interesting thing is that those older, smaller ships had more powerful engines…
A problematic thing is they way ports generally work, you may generally not book a port for a specific arrival date. You have to rush to get there before other ships, to sit still outside and wait in line.
Not very good for fuel consumption and the environment.