[Symmetry]

The rock in this case acts as extra thermal mass that makes it take longer to reach thermal equilibrium, but doesn't change what the ultimate thermal equilibrium is. Only the configuration of the parts of the surface that can absorb or radiate electromagnetic radiation do that. And because rock is a fairly good insulator we only really benefit from the top layer and if the sun went out we would all freeze in a week or so.

[its-summertime]

it changes the amount of exposed area to release heat back into the universe. if you have a non-negligible amount of compute compared to earth, you are going to be approaching a non-negligible amount of space required to radiate that away, along with all the other costs and maintainability issues

[Symmetry]

The formula for the equilibrium temperature for a sphere in sunlight is

    2 * pi * r^2 * L / (4 * pi * d) * (1 -a) = 4 * pi * r^2 * sigma * T^4

As you can see there are pi*r^2 on both sides of the equation, the surface area to cross section ratio of a sphere doesn't change as it gets bigger and so the equilibrium temperature doesn't change no matter how big the sphere is. (d is the distance to the Sun, nothing to do with the sphere itself).