1. What does it mean?
가슴이 웅장해진다 literally suggests a chest becoming grand. As a meme expression it exaggerates being emotionally overwhelmed by a reunion, showdown, impressive scene, or unexpectedly epic pairing.
Also written or said as gaseumi ungjanghaejinda
2. The nuance
It can be genuine admiration or mock grandeur over something very ordinary. It is figurative emotional language, not a claim about the physical size or condition of someone's chest.
3. Examples
“오래 기다린 두 팀의 합동 무대라 가슴이 웅장해진다.”
Seeing these two teams share a stage after such a long wait feels epic.
“만두 두 종류를 한 접시에 담고 가슴이 웅장해진다고 농담했어.”
I jokingly called it an epic moment when I put two kinds of dumplings on one plate.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.