Why is the youngest sibling usually the thinnest and the best looking?

I've observed this and it seems to be an ongoing pattern. Just as how I am the younger sibling in my family and my two older sibling have always been overweight, where I've stayed thin. Not just my family though, I've seen it in many other people too. Why is this? I've also heard that the older siblings tend to have higher IQs, why that?