There is no such thing as naturally thin.
If you are looking to lose weight, watch thinner people -- they have different habits. Watch at a party what they are eating. How often do you see them hitting up the buffet table?
A few other secrets of thinner people:
They eat breakfast.
They eat several times a day.
They exercise regularly.
You'll often find them taking the stairs instead of the elevator.
There's no need to obsess over diet and weight, but if you watch closely, you'll see that thinner people have different habits that keep them at a healthy weight.