These people are Insulin hyper-producers. While the calorie counting explanation of weight gain works as a simplistic model for most people it does not work for these people. They too readily store fat. Because they too readily produce Insulin. Most people could not be as heavy as these people get. The opposite are people with Type 1 diabetes. They have no ability to gain weight without Insulin injections. Type 2 are people who have taught their body to ignore their own insulin. They are normally fat low energy people with bad diets.
The people who gain huge amounts of weight would be fine if they stayed away from any carbs. Even if they never left the couch, they would lose weight in buckets.
It would also behoove people to pay close attention to how many carbs they are actually eating. For example, if five small peppermint patties contain 55 carbs, do you really want to add those to your other carbs for the day (never mind the extra 250 calories)? A low carb diet is something like 40-50 grams per day.
https://www.fatsecret.com/calories-nutrition/hersheys/york-peppermint-pattie-(mini-size)