Becoming uninhibited with 気が大きくなる

Idiom Explanation: Finding courage with 気が大きくなる

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:: 「夜(よる)は気(き)が大(おお)きくなる」; “Last night I became uninhibited.”; Literal: “At night, my spirit gets big.”
なこ:: 「おーぼっち　おはよ」; “Oh, Bocchi, morning!”; 「？　どした？」; “What's wrong?”

Key Points

1気(き)が大(おお)きくなる means to get bold

気(き)が大(おお)きくなる describes becoming more daring, confident, or uninhibited than usual.
Bocchi is thinking about the late-night confidence she felt while reading about making friends.

2夜(よる)は contrasts nighttime with the next morning

The は on 夜(よる) frames nighttime as the topic and sets it against her current school-morning anxiety.
The point is not that night always has this effect, but that it did for her in that situation.

3大(おお)きくなる describes a change of state

〜くなる attaches to an い-adjective stem and means “become …”.
Here 大(おお)きい becomes 大(おお)きくなる, so her courage swells or grows larger.