<div class="circle">
  <i class="fa fa-flag"></i>
</div>

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

.circle {
  width: 3rem;
  height: 3rem;
  color: #fff;
  display: flex;
  border-radius: 100%;
  position: relative;
}
.circle::before {
  content: '';
  position: absolute;
  left: 0;
  top: 0;
  background: blue;
  border-radius: 100%;
  width: 100%;
  height: 100%;
  transition: transform 0.5s ease;
}
.circle .fa {
  margin: auto;
  position: relative;
}

.circle:hover::before {
  transform: scale(1.4);
}