Please link to this mythical person that can violate the laws of thermodynamics, and gain weight without eating?
Nothing to do with the laws of thermodynamics. The number of calories you consume puts a ceiling on the number of calories that you absorb. The efficiency of absorbing calories from food also varies from person to person (and intestine to intestine).
Similarly, the rate at which people burn calories varies relative both to their inherent metabolism and their total calories burned from activity, and their total calories burned from activity varies based on their muscle mass.
Therefore, the equation, broadly would be:
net calories to fat = (calories eaten) * (intestine efficiency factor) - (inherent metabolism) - (muscle mass)(metabolic activity factor)
We know that people with more muscles need to eat more to keep their bulk up.
We also know that people on amphetamines (which, IIRC, used to be prescribed as diet aids) have higher metabolism.
Therefore, it stands to reason that fat people are likely to gain more weight from eating the same food than a thin person. The ways to fix this are:
-Eat less than the thin person.
-Find some drug to lower intestinal efficiency
-Amp up your metabolism.
-Increase exercise and build muscle mass.
Obviously, the simplest and safest solution is to eat less and move around more. That said, to pretend that all people have the same difficulty gaining or losing weight is just preposterous, as is the notion that all fat people simply eat many, many times more calories than thin people. Yes, many do. But not all.