How Did Gon Become Big?

Gon transformation in Hunter x Hunter Episode 131 – Anger X And X Light, that made his body mature around 20+ years, and most likely made him the most powerful person in the world, could have been achieved by Gon going through 4 of the five stages of grief

five stages of grief

The five stages of grief model (or the Kübler-Ross model) states that those experiencing grief go through a series of five emotions: denial, anger, bargaining, depression and acceptance.

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multiple times.

What episode does Gon become big?

Anger × and × Light.

How did Gon become buff?

Basically, Gon made a sacrifice (not being able to use Nen again) in exchange to reach a condition where he could defeat the powerful Pitou. He reached an age where he would be at his peak power level. The transformation was met by fear from Pitou as his power was of equal or higher standing to Meruem.