My guess is the oversized ship isn't going to be a game-breakingly awesome death star type ship. As a government bonus, it's competing with pretty modest bonuses to other gov types. As such, and because each leader can build one, it's likely to be an expensive vanity project comparable to the Santisima Trinidad, the Vasa and the Yamato class battleships. Like these oversized ships in history, they should be scarily big but not unassailable.
I strongly suspect players to accept the cost of building them wouldn't compare favourably with the same resources spent on lots of smaller ships. In every stream I've seen so far, larger ships get annihilated by lots of smaller ships anyway. (although this may be rebalanced)