While I didn't see you with 〜うちに

よつば:

「しばらく見ないうちにまた大きくなった！」

“You've grown bigger since I last saw you!”

Literal: “During the time I didn't see you, you've gotten bigger!”

ジャンボ:

「お？どこでおぼえたそのセリフ」

“Oh? Where'd you pick up that line?”

Key Points

1. しばらく見ないうちに means during the time I didn’t see you

   • The phrase marks the unseen interval as the time when Jumbo supposedly grew.

   • It does not mean Yotsuba watched him grow.

2. また大きくなった is a stock grown-up remark

   • また means “again”, as if this has happened before.

3. Jumbo notices the phrasing

   • 「どこでおぼえたそのセリフ」 asks where she learned that line.

   • セリフ treats her comment like a borrowed scripted phrase.