Why are retina images automatically produced?

I have a wordpress site that runs Avada theme on siteground hosting. I have recently optimised all the images using shortpixel to improve pagespeed. What I am now finding is that the retina images are slowing my mobile pagespeed, and I cannot figure out how to stop the website serving them. I have tried deleting the retina (@2x) images from the server but they reappear. Does anyone know how to turn them off?