The issue happen because your image size incorrect ratio
There are you can see the top of part image is so large, so you can resize by crop image size to balance.
Addition you can change gap between images by css.
Themes → Customize Code → Assets/section-main-product.css
@media screen and (min-width: 750){
.product__media-wrapper .product__media-list.grid {
row-gap: 0;
}
}
