This is the 5847th most frequent Japanese word.
ふくらむ
To swell, expand, or bulge.
In this sentence, "ふくらむ" is used to describe an emotional state that creates a physical metaphorical image of swelling or growing in intensity.
彼女の胸は期待でふくらむ。
Her chest swells with anticipation.
Here, "ふくらむ" is applied to a literal physical action where the balloon increases in volume.
風船がふくらむのを見ながら、彼は笑顔になった。
Watching the balloon inflate, he smiled.
In this example, "ふくらむ" indicates the growth and expansion of abstract concepts like ideas.