In your CSS you are specifying:
#page-wrap {
width: 960px;
margin: 100px auto;
}
Which, as the parent of the tabbed content, also becomes the width of the content. If you want the width of the content to resize with the window, I would change the width to some percentage of the window width:
#page-wrap {
width: 60%;
margin: 100px auto;
}
You'd also might use the max-width
property to limit the expansion of the content.
I've made an updated version of your CodePen.