The number of fat cells becomes fixed by the end of adolescence. As a child, fatty cells proliferate, but remain rather small. Your daily food as a kid influences how many cells you'll end up having as an adult.

Kids nowadays have a horrible diet of sugary drinks and fatty pseudo-chocolate treats.

Fat cells are very much involved in total body weight control, both through enzymes and by hosting lipid-soluble hormones in their oil droplets. So an adult having a lot of fat cells is at a disadvantage in a McBurger world.