Hi, what you are asking is not possible with this plugin. Because the plugin is responsive, the images must be scaled client side. You are asking for a server-side scaling of the images. Something like this would be possible trough asynchronous calls, but it will be very slow, since for every size a new image would have to be generated and downloaded to the client. Apart from that it will put a huge strain on your server, because of all the images that would have to be saved to the server (using caching), or cause a huge load on your server’s processor and RAM.
I agree, for your case it would be useful to have the images resized on the server, because your website has a fixed width. But this plugin is not right for that, and never will be. I might write a different plugin that does this in the future though, but I have no concrete plans to do so for the moment.
By the way: most modern browsers do a really great job re-sampling images nowadays. All your images look very clean and sharp over here. I checked in IE10 and the latest versions of Chrome and Firefox.