What OP is arguing is that each ship is built to the largest possible size at the time of its construction. There are plenty of external limitations at hand, e.g., the size of docking facilities, canal clearance, etc. that mean that a container ship can’t get larger than X.
Exactly. There's literally classes of ships that are something "max". e.g. Panamax. Panamax ships are literally built to the absolute limit of what will fit through the locks at the Panama Canal. And yes, sometimes the locks get bigger, and the ships follow.
For the original commenter to be wrong you'd have to argue that they've been underutilizing what's possible in the state of the art. Looking at the Wikipedia page, I don't get that impression. It sounds like giant engines and equipment on the terminal side are the main limitations, and I assume those capabilities have increased over time. Maybe the original commenter is wrong, although I highly doubt that cargo technology has been underutilized unless the cost of state of the art is/was truly so astronomical such that it genuinely doesn't make financial sense.
With crude oil tankers decades ago the indicators were the bigger the better financially, so that's what was done, and bigger ships were built and financial gains realized.
It was only proven how big was too big once a few ultra-large had been built, and the point of diminishing returns had been exceeded enough so accurate math could finally be accomplished.
Routine commercial operation has been scaled back decades ago to less than the max.
Less than the max that is physically possible, focused now more accurately on better returns.
