Big people burn more calories even when still. Active people burn more than still people of the same size. Muscular people burn more calories than the non-muscular as they have to feed their bigger muscles.
In sealed room tests fat people use more calories than thin people.
Skinny people who can eat anything they want without putting on weight tend to be very fidgety - they never keep totally still. Just wiggling a leg all day can use a large amount of energy.
Hm, this is interesting. I've never heard that before, but it seems intuitive. Do you have any sources for that idea? (Not that I'm challenging, just curious.)
Many people's body's response to overfeeding is to up NEAT, up fidgetyness. But not always. Your body thinks getting fat is a good thing. Famine used to be common. When dieting almost everyone down-regulates NEAT. On a severe diet you feel like being very still.
Those factors are not enough. The human body has many more important factors in play there, like hormones, intestinal flora, and so on.