.main-about__video {
  width: 510px;
  height: 510px;
  border-radius: 50%; 
  overflow: hidden;
}
.main-about__video video {
  width: 100%;
  height: 100%;
  object-fit: cover; 
}

@media (max-width: 768px){
  .main-about__video {
    margin: 0 auto 20px;
    width: 300px;
    height: 300px;
  }
}