.wrapper {
	position: relative;
	width: 400px;
	height: 400px;
}

.popup {
	position: absolute;
	right: 0;
	bottom: 0;
	width: 0;
	height: 0;
	border-radius: 10px;
	background: red;
}

.bigger {
	animation: bigger .5s forwards;
}

.smaller {
	animation: smaller .5s forwards;
}

@keyframes bigger {
  100% {
    width: 400px;
    height: 400px;
  }
}

@keyframes smaller {
  0% {
    width: 400px;
    height: 400px;
  }
  100% {
    width: 0;
    height: 0;
  }
}