I agree with the part that the scale should not matter. (Except, as Richard said, the age of the universe is the same for everything in it, so the largest structures had relatively not enough time to do anything interesting.)
But I think the atoms and galaxies simply do not have enough internal complexity to do something as interesting as cells or animals. Atoms, because there is little inside. Galaxies, because the things inside them do not interact enough.
I agree with the part that the scale should not matter. (Except, as Richard said, the age of the universe is the same for everything in it, so the largest structures had relatively not enough time to do anything interesting.)
But I think the atoms and galaxies simply do not have enough internal complexity to do something as interesting as cells or animals. Atoms, because there is little inside. Galaxies, because the things inside them do not interact enough.