A small note: a Grand Unified Theory is not a theory that unifies the large and small (presumably you mean something that solves the measurement problem in QM), it is a single theory that unifies the electro-weak force and the strong force, leaving us with just two fundamental forces: the unified one and gravity. Solving the measurement problem is necessary for having a complete picture of the universe, but a GUT is not - it is logically possible for the universe to have several different fundamental forces; but it's not logically possible for the macrocosm not to be reducible to the movement of its constituent particles.