Why did Sri Hanuman not take Virat roop and fight with Kumbhakarna?

Sri Hanuma in Ramayana will stand apart from the rest of the Characters.

He is highly learned, righteous to the core, blessed with supernatural powers, wise, valiant, powerful, fearless but yet humble in character.

He chose to remain in sidelines till called for. When he laed the way, his brilliance sparkled in every action, be it acting as a messenger, an advisor, fighting as a warrior, etc, he executed. He did not intervene when others are fighting their battle.

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Though Sri Hanuma offered to take her to Sri Rama, she did not want Sri Hanuma to take her to her husband.

Sita understood the strength of Sri Hanuma and that he can slay Ravana and his followers all alone.

Her husband has to fight the battle and win her in the battle.

कामम् अस्य त्वम् एव एकः कार्यस्य परिसाधने |

पर्याप्तः पर वीरघ्न यशस्यः ते बल उदयः ||

"O Hanuma, the annihilator of enemies! You are yourself surely sufficient single-handed completion of this task. Your elevated energy is creditable indeed."

शरैस्तुः सम्कुलाम् कृत्वा लन्काम् पर बल अर्दनः |

माम् नयेत् यदि काकुत्स्थः तस्य तत् सादृशम् भवेत् ||

"But, it will be appropriate for Rama, who destroys the hostile forces, to take me with him, after routing Lanka with his arrows."

तत् यथा तस्य विक्रान्तम् अनुरूपम् महात्मनः |

भवति आहव शूरस्य तत्त्वम् एव उपपादय || (Sundara Kanda 56 Sarga 3-5 Slokas)

"Therefore, you act in such a way that the high-souled Rama, valiant in battle, may exhibit prowess worthy of him."

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If he intervenes, their glory diminishes and ego gets hurt. Sri Rama and Lakshmana had to toil. After all it was their battle. He helped them by remaining in sidelines.

That is why Sri Hanuma did not try to

• suggest some supernatural task through which he gets the name, but not Sri Rama. Sri Rama had to search for alternatives and it is Vibhishana, who advised Sri Rama to pray for Ocean God and get a passage to Lanka
• Sri Rama prayed for 3 days and 3 nights and finally it was the Ocean God, who told to construct a bridge with the help of Nila, the son of Viswakarma
• stop, though being capable, Indrajit when he was attacking, staying invisible by using his ascetic powers, Sri Rama and Lakshmana.
• Stop Kumbhakarna

And, when really his intervention was called for he went and brought Sanjivani hill for saving Lakshmana.

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Sage Valmiki was giving hint through this episode that the God will not help those, who will not help themselves.