stateful.css

nav {
  display: flex;
  align-items: center;
  justify-content: center;
  background-color: aqua;
  width: 20rem;
  height: 20rem;
}

.container {
  background-color: darkorchid;
  width: 10rem;
  height: 8rem;
}

nav.onCollapse {
  width: 4rem;
  height: 4rem;
}

nav.onCollapse .container {
  width: 2rem;
  height: 2rem;
}