Someone said to me that our bodies stop producing fat when we are very young. He said that once we stop producing fat cells, we'll have the same # of fat cells throughout our life, and when we gain weight, those cells just swell up. We can't create new fat cells.
Exception: He said that women, when they're pregnant, can change the # of fat cells in their body...
Other than that, we're stuck with what we have.
Sure, we can loose weight, and decrease our bf%... but we still maintain the same # of fat cells. Thus, some people are pre-disposed to be fatter than others.