Full-Size images only? Woocommerce

Hello,

when testing my website speed through pingdom.com, I realized that woocommerce loads only the uploaded full size images though I reduced the image size in the woocommerce properties by two thirds. I double checked it with firebug and other developer tools. So there are no thumbnails shown. I regenerated my images using the regeneration tool, still no effect on the problem.
Does anyone have an idea what causes the problem?