Going on my page by phone was generating retina images no matter if the optioon was on or not.

I did some investigating and i think I found out that the check for the retina option in the imageReplacer() function in mavikthumbnails.php in the plugin is not there at Line 186.
(at least I fixed it for me there)

Maybe I have some misconfiguration somewhere else.
Anyways, if anyone has the same problem, maybe this helps.

Fixed it with:

$retinaEnabled = $params->get('retina');
        if ($retinaEnabled) {
            $this->ratio *= JFactory::getApplication()->getUserState('mavikthumbnails.display.ratio', 1);
        }