What I can tell you is that, according to the literature and the experience of millions of people:
- giving adequate protein and some stimulus (i.e., resistance training), even when diets are very restrictive (think 75% of basal metabolic rate), lean mass is largely preserved.
- if you read the Ancel Key's "starvation experiment", every single person lost weight according to predictions. To the point that when subjects did not lose weight according to the prediction, it was because they were (if I remember well) scavenging for food when walking outside. See also the more recent "potato experiment" or try the "burger experiment": two big mac with medium fries and diet soda per day (and nothing else) and a man 200+ pounds will lose weight (the two combined are around 1600-1800 calories).
- people were thinner in the past largely because of greater caloric expenditures (manual jobs, fewer cars, more time spent outside the house) and less calorie-dense and palatable food.
- bodybuilders or gym enthusiasts who are PED-free are normal people who track the calories and macro-nutrients and consistently provide stimulus to their muscle tissue in the direction of hypertrophy. They are not "special people". And while 5% body fat can be maintained only for a short time, 10-12% can be kept indefinitely, from a physiological point of view. Not from a culinary point of view, though, in the sense that you cannot have 3 portions of macaroni and cheese at lunch and dinner.
- there was a recent study on the metabolic differences of thin and less thin people. Thin people eat less, which is quite intuitive. It is very easy to consume calories nowadays: 200 for a snack, 300 for another, let me grab 4 chips and other 150 calories are coming in. Over time, they add up due to arithmetic and one finds themselves fat, while saying: "but I am not eating much!". Overeating is easy, and moving around less than in the past is easy as well. I was with a thin friend at lunch and I was ready to eat like the proverbial pig (say, 1000+ calories), and he ate toasted bread with some ham (maybe 500 calories). Big lesson there.
I recommend the books (and writing in general) of Lyle McDonald.