Finding courage with

()(おお)きくなる means becoming bolder or more confident, sometimes enough to act recklessly. Alcohol, a recent win, or encouragement from a group may produce this temporary confidence.

Examples

Becoming uninhibited with 気が大きくなる

Learn the meaning and usage of the idiom 気が大きくなる in Japanese, with examples from manga.
ひとりぼっちの○○生活 » Volume 1 » Page 22

Bocchi was up late reading a book on how to make friends. At the time, she was overly enthusiastic, but at school the next morning she finds her social anxiety has returned.

ぼっち:
(よる)()(おお)きくなる
“Last night I became uninhibited.”
なこ:
「おーぼっち おはよ」
“Oh, Bocchi, morning!”
「? どした?」
“What's wrong?”