Calories in - calories out is true, but people often have the wrong idea about it. It’s very much not ‘what you eat’ - ‘exercise’. Like you say, being a tall young man probably indicates a decent base calorie expenditure.

Most people are aware that maintaining muscle mass spends more calories than maintaining fat stores (whose purpose is to provide calories during a famine!) but for many people the main uses of the calories they consume is to maintain body temperature. There’s indications that who are obese maintain a lower core temperatures during the day, which uses up less energy.

How many calories are spent on maintaining body temperature varies between individuals by up to 600kcal. So it possible that you and I eat the exact same diet, and you go for a two hour walk every evening while I scroll my phone on the couch, and I’ll lose weight and you’ll gain it.

And even in a sedentary lifestyle there’s a large variety of energy expenditure. Studies on esports are not conclusive yet, there’s some pretty dumb stuff sponsored by gaming companies that say shit like “one hour gaming = 1000 situps”. But more reputable studies suggest a more modest 50-100% increase in calorie expenditure compared to just sitting (so, the same as standing up or perhaps a gentle walk). And just fidgeting, changing posture and shifting in your seat can use up 350kcal

So it’s perfectly possible for OP’s brother to not do much obvious activity, and still use up the equivalent of a couple of big macs or 7 cans of cola, compared with a less fidgety friend with a more ‘efficient’ metabolism.