When you gain weight, your body fills up the existing fat (adipose) cells. When those are full, your body begins to create more cells. People who have been skinny their whole life therefore have way less adipose cells than someone who has been large. Once a large person loses all their weight, it basically just "deflates" the adipose cells they have, but doesn't usually destroy them. Therefore when you have been large and you get skinny, it's easier to gain weight back because you don't have to wait for your body to make new adipose cells to store fat since you already had them once upon a time.
