I think what she means is 'No one can love you more than I'. Even though both that and the actual title line are grammatically legitimate sentences, they have different meanings. The pronoun at the end of each is what is called an 'elliptical phrase', a phrase implied but not spoken except for the pronoun. The Pronoun 'I'' must be the subject of an elliptical phrase when it occurs in one, while 'me' must be the object of any elliptical phrase in which it occurs. So 'No one can love you more than I' means 'No one can love you more than I love you'. 'No one can love you more than me' has to mean, if used correctly, 'No one can love you more than he or she loves me.' The latter would be a pretty preposterous statement, albeit grammatically valid, and certainly doesn't fit the context of the song. So surely what the line really means to say is 'No one can love you more than I,' which does fit into the song's context.