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As usual, weight is driven by Calorie in - Calorie out. If it’s positive you gain weight, if it’s negative you loose weigt.
Your brother is a tall, young male, all of that would point toward a relatively high calorie out also without knowing him in details it’s hard to evaluate. There is always that person taking desert twice when out, but skipping lunch on regular day while doing a lot of sport.
C’est pas un peu contradictoire ? Surtout vu l’état déplorable des finances publiques