It's really a case-by-case basis.

Hunger is regulated by a complex set of hormones produced in the stomach / intestines that then interact with multiple brain regions (pituitary gland. hypothalamus). It's a complicated system and it's influenced by individual genetics. People also burn different amounts of energy at rest, which is also influenced by genetics (eg. FTO gene is associated with obesity bc ppl with certain alleles burn less). People's body compositions are also different. Food habits (do you eat past fullness? do you snack? etc) are also highly influenced by upbringing.

Leading to my point: some people genuinely just don't have much hunger, and/or tend to store less fat, and/or have a different pscyhological response to being hungry. "Naturally" skinny people also tend to unconsciously adjust to having a big meal by eating less the day after. Many skinny people really are that way without any effort.

But there's also quite a few skinny people that go to a LOT of effort to be skinny, and for some of these people, it really is disordered (not necessarily a diagnosable condition, but taking up too much of their daily mental effort to be healthy). I don't wanna get into this too deeply (and I don't have time lol), but back in 2018-19 , I put a looooot of effort into getting skinny, and I certainly succeeded on that. But it took most of my mental energy, every single day, because "really skinny" is not a body type that I can sustain healthily due to genetics or upbringing or whatever (which I only appreciate now). At 45kg (99 lbs) I was still technically on the cusp of a "healthy" BMI but I was mentally and physically unwell. My bloodwork was absolutely whack.

During that time I was on a heap of intermittent fasting / dieting / "progress pics" forums and I found that I really wasn't an anomaly, at least among the people that frequent those types of places - eg. a lot of skinny people with an outwardly healthy diet said they forced themselves to do "water fasts" (ie. eating/drinking nothing but water) for a day or days after eating out with friends. So people will be like "how can you eat hamburgers all the time and be so skinny?? you're amazing!" but not realise what they're putting themselves through when others aren't around.

So there's a dichotomy: some skinny people are in the first group I talked about, and their body type is actually realistic for them because it doesn't take too much effort. But for other people, being very skinny is unrealistic because they will devote so much time/energy to it that it becomes unhealthy.

Edit: as for VERY skinny body types - idk. Some will probs be that way "without effort" but many won't be. Our bodies have a whole lot of regulatory mechanisms and feedback loops that usually don't let us get signficantly underweight if we can help it.

I'll add an option for that.

Bottom line, I agree with you that there are a lot of factors that go into being skinny both genetically, mentally, and effort-wise. I'd also like to say that it probably depends on your definition of skinny. Because tbh (going off of a chart like this link) I personally still see something like 25% or even 35% on the skinnier side. Of course that also kind of depends on the persons height and stuff because different bodies distribute weight differently. That alone is why it's hard for me to say that one body type or another is unrealistic/unachievable.

I've seen it argued that people in the top row are have totally unachievable lifestyles (which isn't the case for everyone) and people in row two have unhealthy body types (which is also not necessarily true). For some people they are able to and naturally achieve one of these body types. So the way I see it, if people can achieve a certain body type then the body type itself isn't unrealistic or not natural. I don't think that it's fair to tell these people that their bodies aren't normal. Of course it's also bad to assume that anyone in row three is automatically unhealthy.