To make the ship management faster and less time consuming it was necessary to turn the fleet capacity into so-called
slots :

How many slost each ship has you can find here:

shipslot

Once you are able to purchase vessels that occupy 2 or 3 slots these will have a much higher capacity equal to the capacity of one-slot ships multiplied by 2 or 3 respectively.

Please check this basic example:

Your Main Dock has maximum fleet size 20 slots:

Ship A – takes 2 slots , double efficient
You have 20 slots (= capacity generated by your Main Dock) – if you use ship A, you will have 18 more free slots (because ship A consumes 2 slots) and there will be 39 ships in your Main dock.
Yet! The ship A be efficient as 2 ships.

This also means that if your Main Dock enables you to use 20 one-slot ships and you will add ten 2-slot ships to your fleet (because under those circumstances, you cannot add more), the capacity of your fleet will be high enough in relation to your level but you will have to manage only 10 ships which will spare much time and the faster the ship management is the faster your profit raises supposing you need half the time you once needed to send out your entire fleet.