Sorry for my late reply and thanks a lot for your message and also the nice comments :)
I don't own a Nexus 7 so it is going to be hard to try it for me directly. But let's start with the basics: using the server-method, if you turn on the DEBUG mode in the plugin settings, then load the page with your standard computer, are the images returned much bigger? (you'll have to use the Chrome Developer Tools for that for example). I just wonder if the server-method works on your website, so if you have an iPad or a Retina display available somewhere maybe you can try.
Then if this is not working, then there is a problem with the plugin using the server-method on the Nexus 7. Something that we can try is to modify the high-res screen detection in the server side to match the one found in the client-method one. Very easy to do! Just replace the file "retina-images.js" with this:
// Retina Images from http://retina-images.complexcompulsions.com/
// Modified by Jordy Meow (inspired by the retina.js script).
var root = (typeof exports == 'undefined' ? window : exports);
document.cookie='devicePixelRatio='+((root.devicePixelRatio === undefined) ? 1 : root.devicePixelRatio)+'; path=/';
Let's see if it works :)