膨らむ

First cited to the mid-800s.^[1]

From 膨, 脹 (fukura, “a bulge, a swelling”) +‎ む (-mu, suffix indicating “looks like, seems like”). The fukura portion is in turn from now-archaic verb 膨る, 脹る (fukuru, “to swell, to bulge”), superseded by modern 膨れる, 脹れる (fukureru) of the same meaning.

• (Tokyo) ふくらむ (Heiban – )^[2][3]
• IPA^(key):

膨(ふく)らむ • (fukuramu) intransitive godan (stem 膨(ふく)らみ (fukurami), past 膨(ふく)らんだ (fukuranda))

1. to swell, to expand, to get bigger

The intransitive verb 膨らむ (fukuramu) is used to describe something that happens naturally of its own accord, generally with a positive sense.

• 蕾(つぼみ)が膨(ふく)らむ、夢(ゆめ)が膨(ふく)らむ、胸(むね)が〔幸(しあわ)せなどで〕膨(ふく)らむ

  tsubomi ga fukuramu, yume ga fukuramu, mune ga 〔shiawase nado de〕 fukuramu

  the buds swell, one's dreams grow bigger, one's chest swells

The synonym 膨れる (fukureru) is used to describe something that happens due to an abnormality or external force, not necessarily with a positive sense.

• パンが膨(ふく)れる、躯(むくろ)が膨(ふく)れる、頬(ほお)が膨(ふく)れる

  pan ga fukureru, mukuro ga fukureru, hō ga fukureru

  the bread rises, the corpse bloats, one's cheeks puff up (→ one pouts or sulks)

• 膨らめる (fukurameru, “to inflate or expand something”, transitive)
• 膨らます (fukuramasu, “to make something swell or expand”, causative / transitive)
• 膨らみ (fukurami, “a bulge, a swelling”)