Is this intended? Cuz I’m pretty sure in real life destroyers are the bigger ship
Yes, it is intended. Consider it a gratuitous design decision. It has zero bearing on whatever Earth’s navies chose to do, and should not be mistaken as basis for debate past this point. 
Please do not expect GSB2 to mirror all details of all aspects in our earthbound military history. If you are looking for “realism at all costs,” you will be disappointed; it’s not that kind of game.