[thrw42A8N]

It's not so simple... I eat whatever I want and struggle to get my weight over 80 kg, which would be a healthy weight for my height.

[kulahan]

Nobody is implying it’s so simple that you can “eat whatever you want” and be at a healthy weight. This is true if you’re underweight as well. If you’re trying to gain weight, you need to eat more than you want.