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.
https://en.wikipedia.org/wiki/Ghrelin
https://en.wikipedia.org/wiki/Leptin
TLDR: The body has a setpoint for weight which is tuned to environmental conditions and will attempt to reach it by modulating hunger.
It takes 8-12 months for these hormones to reset, so any diet is ineffective unless permanently maintained.
High setpoint? Prepare to be ravenously hungry all the time if you eat below your hormonal baseline.
You can also apparently wear weights to lose weight for this reason.
https://www.thelancet.com/journals/eclinm/article/PIIS2589-5370(20)30082-1/fulltext
That finding is pretty mind blowing!