Maybe Bonbi got fat from Eating disorder.
An eating disorder is a mental disorder defined by abnormal eating habits that negatively affect a person's physical and/or mental health.
https://en.wikipedia.org/wiki/Eating_disorder
https://adaa.org/understanding-anxiety/related-illnesses/eating-disorders
https://www.healthline.com/nutrition/common-eating-disorders
Taylor Swift also got fat from eating disorder back in 2017-2018
https://www.theguardian.com/music/2020/jan/24/taylor-swift-eating-disorder-miss-americana-documentary-sundance-film-festival-lana-wilson