That's not how space or terrariums work. A terrarium does not spontaneously produce energy out of nothing, it gets energy from the sun. Heat input from the sun is proportional to cross sectional area, while heat loss to space is proportional to surface area, which scale the same for a sphere. A larger object will have more thermal mass which would make it take longer to change its temperature, but it will still have the same thermal equilibrium. Terrariums do not need to be spheres, so the volume does not necessarily scale as the radius cubed.