This goes both ways. My BMI is on the low end of "normal" so not technically underweight anymore, but I was really skinny as a kid. I was one of those skinny people who claimed to eat a lot. "My metabolism is just super fast" I would say. I hear other skinny people use this excuse all the time.

Then I actually started tracking my calories and realized that all those people telling me to eat more were right. I had been starving myself without even realizing it, because that small amount of food was just what I was used to.

In some ways I think gaining weight is actually harder than losing weight. Refusing to eat when I feel hungry is a lot easier than forcing myself to eat when I already feel full.