I used to feel critical towards people who I felt 'weren't trying enough'. Over the years, I've come to realize people have different metabolisms. Different genetics. Different appetites because of genetics. Not saying watching what you eat is bad. Just saying embrace what you got, and what others got, and enjoy your life.
I'm one of those people with a slow metabolism -- I can put on weight by merely thinking of food! -- and you are so right. I've just returned from three weeks in an Indian ashram, where everyone eats exactly the same. The main staple is white rice, which I never eat at home. But I ate the same as everyone else, this time; but I always asked for half-servings of rice. There's also a lot of sugar there: in all hot beverages, and in the deserts. What I noticed is this: many of the people (Indians) who live there permanently, are bone skinny, even if they eat mountains of white rice (which they all do!). One or two could be classified as overweight. No-one is obese. No-one does much exercize. I walked half an hour up a mountain and half an hour back every day. I gained two kilos in three weeks (Actually, I thought I would gain much more; I was happy to see that the trousers I wore for the flight out still fit me on the return flight!)
I also have to add that low-fat is not a recipe for losing weight. It's eating the right fats, and the right carbs. I lost the most weight in my life EVER, when I switched all my fats around -- mostly, no more processed fats (sunflower oil and co), just olive oil and butter and, especially, lots of coconut oil and whole carbs. So, very high fat diet -- and still I lost 20 pounds in a month, and kept it off.
I don't think being skinny has anything to do with it. Food itself, does. A good, healthy diet with lots of fresh fruit and vegetables will keep your mind keen and alert, and that's the main secret to a good creative output. Junk food and co will make your mind sluggish and slow -- Check out the Indian theory of Sattvic versus Tamasic foods, and their effect on the mind. (Too lazy to google it myself right now.) (ETA -- OK, I just did -- here's some basic, if a bit simplistic -- info:
http://ayurveda-foryou.com/treat/foodtemparament.html)