To expand, swell, or become inflated outward (intransitive). Refers to something increasing in size or volume.
風船が空気でゆっくり膨らんだ。
The balloon slowly inflated with air.
彼女は頬が膨らんで不機嫌な顔をした。
Her cheeks puffed up and she made an annoyed face.
期待が膨らむばかりで、現実はまだ見えていない。
Expectations only continue to grow, while the reality is not yet visible.
発酵させると生地が膨らむ。
When you let it ferment, the dough swells.