I think you’re misunderstanding how wordpress creates these thumbnails. It doesn’t just create them on the fly when the page loads.
When the featured image is uploaded in the admin, whatever size it’s uploaded at, is what it will display on the site as. If the image you upload is 1000px by 1000px, and the thumbnail on your site is 150px by 150px, it is STILL going to deliver a 1000px by 1000px image to your site regardless of what css is in place. You can use css to make it look 150px by 150px, but it’ll still be a 1000px by 1000px image.