Because supermodels are human beings, they lose weight just like everyone else when they need to. They have to consume less calories than what they take in. Which depending on their lifestyle is not that difficult to do.
First lets understand there is a difference between losing weight and looking good. Not all models have to lose weight if they are living healthy. But perhaps, during that time of month, a model ate too much ice cream and brownies to subdue her cramps and it has gotten to her hips or abs. She will want to lose that off.
Simply eating less than what the body will consider extra calories, which will be converted to fat, is all it takes. You could decrease the intake or increase the physical activity done per day. There are more supermodel who resort to exercise like yoga, Pilates, or running (Gigi Hadid does boxing), in order to stay fit and healthy.