html, body {
  height: 100%;
}
body {
  display: flex;
  justify-content: center;
  align-items: center;
}
ul {
  display: flex;
  
  width: 100vmin;
  height: 50vmin; 
  
}

li {
  flex: 1;
  transition: .5s;
  background-size: cover;
  background-position: center center;
  opacity: 1;
  
  &:hover {
    flex: 2;
    transform: none;
  }
  
  &:nth-child(1) { background-image: url(http://scontent-a-iad.cdninstagram.com/hphotos-xfa1/t51.2885-15/10735084_682299368535661_1815478379_n.jpg); }
  &:nth-child(2) { background-image: url(http://scontent-a-iad.cdninstagram.com/hphotos-xap1/t51.2885-15/10809853_1495259367419434_426351778_n.jpg); }
  &:nth-child(3) { background-image: url(http://photos-d.ak.instagram.com/hphotos-ak-xap1/10748001_601113343325931_1829449078_n.jpg); }
  &:nth-child(4) { background-image: url(http://scontent-b-iad.cdninstagram.com/hphotos-xfp1/t51.2885-15/10784935_866349820076780_1585696996_n.jpg); }
  &:nth-child(5) { background-image: url(http://scontent-a-iad.cdninstagram.com/hphotos-xfp1/t51.2885-15/10802569_979394625409535_1915824306_n.jpg); }
}