膨 rarely stands alone in modern Chinese — it almost always pairs into 膨胀 (péngzhàng, 'to expand/inflate', literal and economic) or 膨大 (péngdà, 'to bulge'). Learn the compounds first; the bare character mostly appears in technical or older writing.
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The radical 月 (here standing for 肉, 'flesh') hints at the original sense — flesh that is 'puffed up'. Compare 胀 zhàng (also 月-radical) which is its frequent partner in 膨胀.
