If you think about it, however much you scale the box & spheres, the ratio of sphere volume to box volume is never going to change. Double it, you double both volumes, so the percentage is the same.
This means it's going to cancel out. The volume of the spheres will be some multiple of r cubed, and the volume of the spheres should be some multiple of r cubed. r cubed/r cubed = 1, so it's just the ratio of the multiples.
It's also exactly the same as one sphere in a box, as the two "sides" of the box are identical, so have the same ratio.