Configure image size
In case: Images are being requested on 300x300 while on the configuration you set 1200x1200 which is the full image size for your images.
When Images size configuration on theme settings doesn't work, you should check in your admin site because maybe you set config product image dimention/Grid mode => 400x400
And If you want the product image will be 1200x1200, you can change the config in admin
But please note that if the image too large, It will slow down the page load speed.