How could Banquo be “lesser than Macbeth and much greater” and “ not so happy, yet much happier’’? act 1 scene 3

This has to do with the motif of equivocation that is so evident throughout the story. Equivocation is the use of ambiguous language to conceal the truth. The witches are merely trying to mess with Macbeth's head. The truth lies somewhere in-between. We can interpret both statements of having shades of truth in the rest of the play.