.product__image-main {
  cursor: pointer;
  max-width: 500px;

  width: 100%;
}
.product__image-main img {
  height: auto;
  transition: transform .8s ease; /* ゆっくり変化させる */
}
.product__image-main:hover img {
  transform: scale(2.0); /* 拡大 */