To keep your body at a certain temperature (higher than air around you) you need energy. You have a mass, and to move from point A to point B you need energy. Your cells are constantly being replaced, and there is a lot going on inside you. All this takes energy, and it's pretty predictable how much it will take. Now genetics play a role. It dictates where fat is deposited in your body. Whether to burn fat or make you lazy if not enough energy is delivered, etc. But it won't break basic laws of physics. You won't be able to maintain 100kg of weight while on 2000-2500 cal diet. And with balanced meals this is enough to not feel hunger.
You are underestimating the "efficiency" the body can pull to use less calories.
I have thyroid disease, and I gain weight easily, I am used to eating little to no food (in the literal sense, because of other problem I have, unrelated to the thyroid disease, I can forget to eat and sometimes don't eat for an entire day or two), people always ask if I am on a diet even when I am just eating what I eat normally, also I love restaurants that charge the food per weight, because it saves me lots of money.
Still I am 130kg right now, and struggling to get any lighter, my body just figured how to be very efficient, for example my temperature is lower than normal (the lowest I measured in a "healthy" day was 35 degrees), the muscles related to walking are stronger than everything else (when I go to the gym, exercises that use those muscles bore me, I can keep using more and more weight until it is dangerous to the integrity of tendons and bones), while all my other muscles are very weak (my little sister can carry weight much more easily than me for example).
And also my body don't "waste" energy maintaining certain parts of it, for example hair (my hair is constantly breaking), nails (they grow slow and shatter easily when I try to trim them), muscles (the only kind of exercise that makes me stronger is very low rep high weight exercises, cardio exercises make me lose lean weight very fast instead).
Also one of the first persons to notice I had a thyroid problem was my dentist, I had a jaw problem that had to be fixed using braces, she noticed the progress speed on my case was odd, my healing speed was weird.
My parents dog accidentally scratched me with her claws two months ago, it was a very superficial thing, I only noticed later (it was not painful when it happened), it still don't healed properly.
Although I am a sort of extreme case (sort of, because there are people with cases that are much worse than mine), I can tell you it is very possible to keep a 100kg body with 2000 cal diet.
while I agree there is a lot of variation in caloric maitenance requirements, and OPs comment was overly broad, having a thyroid condition makes you an outlier and not something we can base the response of the average individual on.
Everything else you said is observational and irrelevant when comparing yourself to another individual as there are many many things that could explain those differences.
