Several factors influence the type of ships a seaport is able to handle.
The first factor that influences the type of ships a seaport is able to handle is the depth of the water around the port. Deep water ports tend to be capable of handling bigger ships than shallow water ports. Of course, shallow water ports can be dredged, to make them deeper, but there is a limit to the level at which this can be done in a viable manner.
The second factor that influences type of ships a seaport is able to handle is the infrastructure available at the port. Ports with poor infrastructure are unable to handle big ships, even if they happen to be located in very strategic natural harbors with very deep waters around.
The third factor that influences the type of ships a seaport is able to handle is the manpower available at the port. This is both the management manpower and the manpower that provides the labor. It is possible for a seaport that is very strategically located, very deep and very well equipped in terms of infrastructure to end up being unable to handle big ships on account of mediocre management or on account of the necessary rank and file workers not being available.