My brother, 21 yo, is like this. He’s ~180 cm tall and has never weighted more than 61 kg (during the 2020 covid lockdown, he made a big deal out of this since it was the first time he passed the 60 kg mark; after the lockdown was over he immediately dropped back to his perpetual weight of ~55kg). I live with him so I know his eating habits: he eats all the time, whatever he wants, whenever, how much he wants and simply doesn’t put on any weight. Is very energetic, no sick look / demeanour. It’s just fascinating to me cause I’m not like that at all, I have to watch what / how much / when I eat.
The people “not gaining weight no matter how much they eat” often aren’t eating as much as you think. I’ve heard people say this of me too but they just don’t realise how often I skip meals. I regularly eat just breakfast and nothing else the entire day. Yeah, you might see me destroying 700 grams of candy at one sitting but what you don’t know is that’s also the only thing I’ve eaten all day.
I was the guy people used to think about like OP’s brother. When you see me hog down, it’s because I haven’t put a single calorie in my mouth up until then.
I was like this and was eating a shit ton so some people at some point in their lives have the super metabolism.