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.
