Heat isn't produced by the volume. Heat may be produced by something within the volume, but it's not the volume's existence that causes heat to be produced. There is no fundamental reason a bigger terrarium should produce more heat, nonetheless that heat production should be directly proportional to volume.

Yes, obviously it'd be the stuff in the terrarium rather than the space it occupies that produces heat, but the amount of stuff you can fit in it is determined by the occupiable space. And if that stuff is producing heat, such as by decay, there's going to be more heat with more stuff. Though even if it cooks itself for a while, it should eventually settle on a temperature determined mostly by orbital parameters and material properties rather than size, since the stuff can't be net exothermic forever. But greater atmospheric depth probably still increases equilibrium temperature by reducing heat transfer through that side of the terrarium.