Notes: If it is cost-efficiency, then either small or large ships probably have an advantage in combat effectiveness per unit cost. In which case, you'd probably only build the more efficient ship.
IRL ships of varying sizes have varying specialized roles that suit their size. For instance, torpedo boats were invented as a short-range very combat cost effective way to deal with big ships. This would have resulted in the end of big ships being able to attack other countries, except that they invented a small type ship to destroy torpedo boats, which is where we get destroyers from (the were originally “Torpedo Boat Destroyers” and later became important in submarine detection and destruction).