Adding padding and border sizes will now reduce the content size, but it makes it so much easier than trying to add up border + padding + content of every element in your CSS.
Without the box-sizing: border-box property your aside might be too wide (because of padding, margin, width properties) to fit on the side and be pushed down to the bottom of the page.
Box-sizing does not get inherited, so it does not work if you put it in the CSS body element. It does work in the universal selector:
* {
margin: 0;
padding: 0;
box-sizing: border-box;
}