G'Thar'Du Cannon & Gravi-Damage

Ayo, I wanted some feedback on the G’Thar and how the game calculates the gravitational damage “based on size of the ship” from the nerf. 

I’m thinking, "Hey, let’s use it on the Defiler, that’s a massive ship, and it does little extra damage. What’s the reasoning behind this?

reasoning: ship sizes: 1) interceptor, 2) fighter, 3) frigate and 4) destroyer. Defiler can either fit on destroyer size or “structure” (which gets no resistances and if im correct, double dmg from explotions)