The second reason is that, when designed for any given speed, the large ships have more space available for whatever is to be carried; one 15-knot ship of 20,000 tons normal displacement, for instance, has about one and a half times as much space available for cargo, guns, and what-not, as four 15-knot ships of 5,000 tons each.

These two reasons apply to merchant ships as well as naval ships.

A third reason applies to naval vessels only, and is that a few large ships can be handled much better together than a large number of small ships, and embody that "concentration of force" which it is the endeavor of strategy and tactics to secure.

A fourth reason is the obvious one that large ships can carry larger guns than small ships.
The distinctly military (naval) purpose for which a war-ship is designed necessitates, first, that in addition to her ability to go rapidly and surely from place to place, she be able to exert physical force against an enemy ship or fort, and, second, that she have protection against the fire of guns and torpedoes from enemy ships and forts, against bombs dropped from aircraft, and against mines.
This means that a man-of-war, intended to exert the maximum of physical force against an enemy and to be able to withstand the maximum of punishment, must have guns and torpedoes for offense, and must have armor and cellular division of the hull for defense; the armor to keep out the enemy's shells, and the cellular division of the hull to prevent the admission of more water than can fill one water-tight compartment in case the ship is hit.
It must be admitted here that, at the present moment, torpedoes hold such large charges of explosive that the cellular division of ships does not adequately protect them.

This means that a contest has been going on between torpedo-makers and naval constructors like the contest between armor-makers and gunmakers, and that just now the torpedo-makers are in the lead.