You're missing the fact that tightly controlled feedback mechanisms govern appetite. That's what allows maintaining weight in the real world. Magically add 20lbs (or an apple a day) to a healthy person and they'll feel correspondingly less hungry.
impact on how much calories people spend simply moving their own bodies
Actually, it's mostly going to be the metabolism of the tissue (extra fat tissue needs flood flow, temperature regulation, energy for cellular processes etc too), and that can be significant, although not as much as hunger regulation.
People are forced to eat more than they should primarily because of hunger pangs.
I am not sure this is true in contemporary West. I suspect that a lot of overeating happens because of social cues ("I'm at a dinner party so I should eat even though I'm not hungry") and for purely psychological reasons -- from boredom and activity displacement ("I'd like to procrastinate a bit, let me go and have a snack") to hedonics ("Sugar boosts make me feel better, yay sugar!"). None of that is actually hunger.
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