Unicron is larger than most moons, ingesting entire planets into his system. Besides the fact that unicron was considered indestructable by any method and means save for the Matrix of Leadership, he is also capable of covering great distances in robot mode. Now if we look at the scale of unicron in transformers, the movie, he should be considerably larger than the death star, even in robot mode. Now say all you want about the death star being able to rotate this way and that, but the fact is that unicron has weapons of his own that he could deploy. Unicron is far more maneuverable than the deathstar. Rotation from the death star would have to be near instantaneous, which has not been demostrated in the movies.
