It so simple the math that there are 5'10 people weighing 200 pounds with a BMI of 29 who eat 1700 kcal per day and gain weight, and people with exactly the same body built who eat 4500 kcal a day and lose 6 pounds a month.
Let's call them individuals A and B. Both are male, reaching their 40s with a pretty sedentary life, same build and in good health. Their excuse of physical activity is climbing a ladder to change a lightbulb or pushing the cart in the hypermarket once each two weeks.
Both are overweighted and both are dieting.
Individual A lost 10 pounds (from 210 to 200) in previous weeks by following a 1700 Kcal traditional diet. Lots of fruit and vegetables. No fat excess. Limited consumption or starchy items, etc. Let's say each day he eats 120 g proteins, 200 g carbohydrates and 50 g fat.
He has reached a "barrier". His ideal weight is 157 pounds, and being him one of that 40% of the population who goes up and down in "quanta", his quanta are about each 11 pounds.
As a consequence, his adipocytes have become lighter but they are resisting apoptosis, secreting inadequate levels of leptin and because some adipocytes are dying, the fagocites taking care of them are secreting cytokines with different effects including stimulating a higher level of ghrelin
As a consequence, his brain has received signals to low down his basal metabolism, to be hungry and to solve the problem by looking for food and eating it.
Of course Mr A is a civilized person and he is not going to pay attention to what his primitive body is telling him. A weak will is fatal when diets are involved -he thinks- and he is resolved to persevere.
The final result is his body saving energy. In an unconscious fashion he is moving less so he saves some 200 Kcal a day, but his basal metabolism went down a 30% so he's saving 600 Kcal extra. That is, he started his diet eating 1700 Kcal a day and burning 2300 Kcal, so he start to lose one pound a week during 9 weeks. Then, his problems began.
Now he feels cold. He has turn up the heating. He wears some warm clothes indoors. He is hungry all day long. He's depressed. His nose is starting to drip and he's afraid he's gonna catch a cold.
He doesn't know but after several days of suffering this weight impasse he's going to drop the diet with a very well though leptin-ghrelin driven cerebral made excuse.
[Individual A is "100% belly": he has a bit of man boobs, but his extremities, neck and face are pretty lean]