She has lost only one kilo, indeed less: to be precise,  Adrienne Osuna weighs about 900 grams less than when she started her slimming journey. Still, she wears six sizes smaller. You got it right: six sizes less.
From an American size 16 to a size 10. 
How come? The story of this woman has made the rounds of the American news: six sizes less but less than a kilo of difference compared to the first. The before and after photos leave no doubt: Adrienne Osuna actually lost a lot of weight, due to a different body composition.

But why hasn’t she lost weight?
Simple: Adrienne, a young mother affected by severe obesity, had a high percentage of body fat and very little lean mass, as seen in the photo. She lost six sizes simply because part of her initial weight has “figuratively” converted from fat to muscle: or rather, since lean mass does NOT turn into fat and vice versa. 

So her body composition has changed.

He lost pounds and pounds of fat and gained pounds of muscle at the same time by doing power lifts, then strength exercises, 5 times a week, and 3 cardio sessions per week. Although the difference is only one kilo, muscle has much less volume than fat, so this explains why, despite having lost only a scant kilo on the scale, this woman is visibly leaner.
In fact, she actually lost six sizes. Here in summary we are faced with a healthy case of weight loss, obtained by combining a healthy diet and exercises to promote lean mass and reduce fat.