I think that everybody has their issues with weight gain/loss. I always thought it was sugar but the best diet I was on was a low-fat diet. It fit in ivrit with the saying that ??? ?????, but if you’re eating a low-fat cake that has tons of sugar, that would be a problem and the opposite – a low-sugar high-fat cake won’t do anyone any good in the weight loss department either.
As far as weight loss rate dependent on a person’s weight, muscles burn calories differently than fat, so a more muscular person weighing the same as a less muscular person will burn more calories just by virtue of his muscular physique. But if an overweight person does get exercise, best is aerobic exercise (HR above target) for 20 minutes, then they should burn more calories than a thin person who does the same exercise.
If an overweight person goes on a diet that involves eating significantly less than the calories necessary to maintain his body weight (let’s say 600-700 calories a day), after a few days of this regimen their body will go into starvation mode and won’t release the fat because it feels that it won’t be able to get the nutrition it needs and conserves all calories for survival purposes.