Late but the exact response. Due to the fact that mafic to ultramafic rock composition (Si+Mg, Simatic) of the oceanic crust which makes the oceanic crust more denser and heavier than that of the Sial composition of the continental crust (Si+Al) that makes the continental crust less denser.
As far as the oceanic to oceanic plates' subduction is concerned, it is again due to the mineralogical and compositional variation too. Such an variation can be in peridotitic and or basaltic composition.