Not to make any excuses for anybody, but people and metabolisms are different. I struggled to put weight on when I was young. In college, I never had to worry about what I ate, while others around me got fat. Some people are genetically predisposed to not only put weight on, but accumulate it in certain parts of the body. It’s a fact.
So some people have to work a lot harder to maintain low body fat, while others can live on Big Macs and Chipotle and be skinny as rails.
Tons of studies and genetic data on this, an extreme example being Polynesians who through the process of evolution and natural selection are predisposed to metabolic disease including obesity in modern society. The same traits that hurt their health now, helped them survive and thrive prior to modern industrial society when they went through periods of semi-starvation and long ocean journeys.