How do certain people who dont exercise stay remarkably thin?

they eat remarkably less, plus have a high metabolism.

Depressing how people here get butthurt when you say you're one of the thin ones.

Look, it's genetics. That's just how it is.

I don't eat remarkably less..

Very impressive 1500, 3:45, and based on that (4:03 mile equivalent), you should have been around 13:30/28:00 in today's era, with Supershoes etc, double thresholds....Your 29:05 was not representative of your true worth.

People are not honest with themselves (and others) about how much they eat.

Is the simple answer to the OP

Diet

I eliminated sugar, pizza and potato chips from my diet and I'm as light as I was in College.

You can't gain more than the calories in and you have to lose at least the calories out. That doesn't mean that you will use all the calories you intake and it doesn't mean you might not expend more than you think you should be expending.

This. It is actually mostly calories in / calories out. But if you eat 500cal in vegs or other fiber rich stuff, it will last for much much longer than 500cal in donuts. Cut out the snacks.

You don't sh*t calories unless your gut has a medical condition or you take meds to block fat absorbtion or whatever.

Was skinny as a kid myself (2 min 800). Now just trying to manage and jog a little.

That’s not true, just the very approximate truth that they tell you that’s misleading. It is true that the body is mostly pretty efficient in using calorie sources supplied to it, but there’s a 5-10% variance across the population on average. For some calorie heavy foods like nuts, some people absorb as little as a third of the listed calories (computed using bomb calorimetry), and I too personally find little correspondence between the ginormous calorie labels on nuts and how satiated they make me feel, even as a person whose satiety signals are otherwise pretty accurate maintenance calorie counters.

In addition to the absorption error above, calorie labels have up to a 20% error, NEAT can subconsciously induce a counting error of 15-20%; foods have 3-20% calorie loss in the digestion process etc; all of which make calorie counting not worth the effort for most people.

Pizza? Life is short.

Some people are addicted to food. Kind of hard to break since you also need it to survive.

By analogy with alcohol: some people only drink in social situations. You'll see them drinking and experiencing the effects of alcohol (maybe quite a lot) but consuming alcohol only happens in particular circumstances and otherwise they don't drink at all.

Other people drink regularly on their own. Often they'll feel ashamed and hide how much they're drinking. They might rarely seem drunk. They drink much more than the party drinkers.

When it comes to food, I'm one of those people who'll forget to eat outside of routine circumstances. I'll eat a lot if the occasion calls for it (including meals out with friends etc.) I'll eat three meals a day out of habit. I make a point of eating some extra energy-dense food after a run otherwise I'll lose weight while training. But if my day is disrupted somehow and I miss a meal, the first I'll know about it is that I get irritable and can't concentrate. I took a week off running recently and it threw off my routine (and mood) to the extent that I lost weight despite burning far fewer calories. I don't 'comfort eat' - eating is a bit of a chore, made easier by it being a habit and a social activity.

Incidentally, alcohol is also quite energy-dense. If you drink 140 ml of ethanol per week that adds about 5% to your calorie intake.

Bolded text above is absolutely, 100% FALSE.

There are a large number of calories in human sh!t. It takes an astonishing level of ignorance to not be aware of that fact. Why exactly do you think flies swarm pretty much any kind of sh!t, human or otherwise? Hint: It's to make use of the large amount of calories in the sh!t.

Most of the shat calories are in the form cellulose though, which eg cows could digest, but not humans.

And for the flies, Google is your friend. Yes there's some human usable calories left but mostly other stuff they like... Because flies. Additionally they love to mate in it and lay their eggs.

Diet and genetics

No one is suggesting that humans should be eating human sh!t. Only that there are significant calories in human sh!t.

Therefore your post is irrelevant.

And why do you suppose they like to lay their eggs in sh!t? Wouldn't be because there are large amounts of calories available for their larvae to live on by any chance, would it?