Spyke

Wasn’t it that she had more magical ability than the rest, then got into all the potions that boosted it more, and she was using her magic to make herself grow and become human?

19
lemmy.world

Fujimoto implies that she is the first and eldest of the sisters between him and Granmamere.

16

This is the answer to OP's question... all the other replies are discussing events that happen after we see Ponyo is already bigger than the others.

9

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Why is Ponyo so much bigger than her sisters? | Spyke