4Minute's Gayoon was never fat, but she did lose a lot of weight last year. Being already slim to begin with, she looked somewhat awkward after her 8kg weight-loss back in 2013. In fact, fans said she lost weight because she was recovering from her jaw surgery at the time and couldn't eat apparently.

I found an interview where Gayoon herself stated that she tried the Enzyme Diet and successfully lost weight with  it. Just google it and you may find it very easily.

Before and After:

Right now Gayoon is still thin but this year she voiced her concerns about her weight:

"I had quite a lot of popularity in Brazil when I was plump.  However, after losing weight, that popularity dissipated.  I think I'm going to need to gain weight again."