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When Janie reflect that, "Nanny had taken the biggest thing God ever made, the horizon - for no matter how far a person can go the horizon is still way beyond you - and pinched it in to such a little bit of a thing that she could tie it about her granddaughter's neck..." what she meant was this.
What is Nanny only saw a very narrow field of possibility, and she wanted Janie to accept that tiny view of the horizon and live in it, even if it hurt her to do so. (Answers may vary)