You can’t really conclude anything from one person gaining two pounds in a week and another person not. For one thing, that’s well within normal weight fluctuation. Weigh another day and the results might be very different. For another thing, it’s actually pretty difficult to gain weight that fast. A 1000 calorie surplus is OK for one or two days, but try doing it for seven days in a row. You just won’t feel hungry enough, unless you have some kind of eating disorder.
This, too.
Some information about the most famous overfeeding studies, most of which were unfortunately very short term with small sample sizes.
Results of the famous SIMS Vermont prison study seemed to show that some lean men can be overfed by up to 3000 calories daily over many months with minimal weight gain, need more calories daily to maintain this weight gain (BMR increase), and return quickly to their previous ‘set point’ once they return to a normal caloric intake.
My personal experience seems consistent with that data, but it’s really the only study of its kind and probably always will be.
It doesn’t but it seems that many forget that the basic and easily understood is actually as complex as the rest of your post makes clear. “Calories out” is a more difficult number to get ahold of, a much more dynamic entity, than many think. The concept is basic and simple, the reality is as elastic as the waistband in the pants my Mom used to buy for me back when I was a kid and shopped in the Huskies section. 
I’d also like to echo the comments that being thin does not mean that you are healthy. A moderately overweight individual who eats healthy choices and exercises regularly is much more likely to have better long term health than a thin person with the habits our op described.
And about that overfeeding study referenced by rhub, my second cite in my first post in this thread was one that replicated the essence of that work in 1999 and documented that the major difference between the group that stayed thin and the group that gained weight was the tendency (or lack of a tendency) to increase NEAT in response to overfeeding.
I have read other studies, but probably couldn’t find them online. However the jists of some are on overfeeding studies: Everyone is given X extra calories a day over their maintenance calories (sometimes 1000 a day for 90 days or something). Weight gain varies from 2-30 pounds, and the % of fat vs. LBM also varies wildly.
On underfeeding studies: Same thing, X fewer calories than maintenance and the results are weight loss varies wildly, and the % lost via LBM vs fat varies wildly
Basically, people are really different and even in small studies of a few dozen people you see rates of weight gain and loss that are polar opposites (one person may gain 30 pounds and have all of it be fat if they are force fed 100k extra calories over 100 days, another may gain 5 pounds and have 3 pounds be muscle). The ‘calories in/calories out’ equation really doesn’t take into account the dozens of minor and major biological changes between individuals that can drastically affect how many calories end up going in and how many go out.
I am fat, but I’ll never be 500 pounds. Even if I wanted to be, I dont’ think I could do it. I’ll never be 120 pounds either. But some people naturally stabalize at those weights.
Here’s a relevant article about high fat diets altering brain cell insulation and thus altering feedback signaling:
http://machineslikeus.com/news/brain-cells-shown-determine-obesity-not-lack-willpower
People get fat because of God. It’s His punishment for their lack of moral discipline, obedience to authority, and industrial work ethic.
This explains why Jesus always looks anorexic.
A few years ago there were news reports about a study that showed that people had very different activity profiles, even at desks. Some people tended to stay in one place all the time, others moved around all the time. I’m of the latter, inefficient, variety, and I suspect this is one reason I’m quite think even at almost 60, and even eating lots of stuff - not junk, but lots. I also fidget a lot, and when I walk, I walk a lot faster than most normal people.