So there is padding in the Site Header – you can reduce the bottom padding for mobile in Customizer > Layout > Padding – make sure your in the mobile responsive view.
Likewise you also have the Padding in Customizer > Layout > Container, same as the Site header but reduce the top padding.
To fix the image then go to Customizer > Additional CSS and look for:
.blog .post-image img, .archive .post-image img, .search .post-image img {
width: 100%;
height: 500px;
-o-object-fit: cover;
object-fit: cover;
}
You can reduce the height: 500px;
to 300px or delete that property.