People with bowel issues for example might actually consume vastly more calories than their peers but not gain as much weight due to their body being less efficient in converting caloric intake into fat. People with a fast metabolism might burn more calories per day at rest than others as well, and some of the complex interactions could involve interactions with caffeine.

If people are gaining weight rapidly it's always a good idea to look at the whole picture rather than just caloric intake because that can mask the true underlying causes.