I’ve seen quite a few people for whom keto alone, eating as much as they want, they were able to get thin again. One guy I followed on Twitter got built (muscular, not much fat), and lost 80 pounds, while eating around 100 grams of carbs per day. If he ate more, he gained weight; he just ate fewer carbs and lost weight.
My theory is that those of us who can’t do this, we damaged ourselves more than someone like him. It’s like Ted Naiman, he got ripped by eating low carb, but he was never fat. He looked a lot worse while being a vegetarian, but he never gained 100 pounds. Similarly, the guy I just discussed, I assume he was insulin sensitive even while heavy, because a lot of people here wouldn’t be able to modify carbs that way.
And I’m sure genetics plays some role. For instance, Asians that gain not much weight are much sicker supposedly than are Europeans at the same weight gain. In other words, a European (like me, 100% European, 75% Eastern European) can gain 50 pounds and even be relatively healthy, but an Asian that gains 50 pounds will be way sicker, supposedly. (Yet one more reason BMI is useless.)