Yeah that widget area is full width, but since you’re using a Featured Page widget it’s showing up with a 300px width. You can change this one line 1412 in your css –
.home-feature-bg-alt .featuredpage .page,
.home-feature-bg-alt .featuredpost .post {
float: left;
margin: 0;
overflow: hidden;
padding: 0 0 5px;
width: 300px;
}