膨らます
ふくらます · fukuramasu · JLPT N2 · verb-godan
to inflate
🎈 膨らむ (to swell, intransitive) + causative す = to make something swell, i.e. inflate it
膨らむ means to swell on its own. Adding the causative suffix らす (written as ます here) makes it transitive: someone or something causes the swelling, as when inflating a balloon.