Is that they all of different ship sizes like in galciv.

But I rarely want to use any size other than the largest available at the time.

The main reason for this in Galciv is engine size.

It does not take the same engine to move a shuttle as it does to move a cruiser.

or in modern terms a speed boat does not have the same engine that an aircraft carrier does. 

The speed boats engine or engines are smaller than the carriers but it moves a lot faster.  I know in ftl all ships would have the same max. speed.  But the engines on smaller ships wouldn't have to be as big as the large ships.

This has something to do with mass.  Actually this has everything to do with mass.