Tā xiǎoshíhou shì ge pàngdūn, zhǎngdà hòu shòu le.
He was a chubby kid, but slimmed down after growing up.
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胖墩 is informal and often affectionate, especially when referring to children. It's not as rude as directly calling someone 胖子 (pàngzi). The erhua variant 胖墩儿 (pàngdūnr) is common in northern dialects.
